native gastropods and introduced crabs: shell morphology

NATIVE GASTROPODS AND INTRODUCED CRABS: SHELL MORPHOLOGY
AND RESISTANCE TO PREDATION IN THE NEW ENGLAND ROCKY
INTERTIDAL ZONE
BY
SARAH JOANNE TECK
B.A., Middlebury College, 2001
THESIS
Submitted to the University of New Hampshire
in Partial Fulfillment of
the Requirements for the Degree of
Master of Science
in
Zoology
December, 2006
This thesis has been examined and approved.
___________________________________________
Thesis Director, Dr. Larry G. Harris, Professor of Zoology
___________________________________________
Dr. James E. Byers, Associate Professor of Zoology
___________________________________________
Dr. Thomas D. Lee, Associate Professor of Forest Ecology
___________________________________________
Dr. James T. Carlton, Professor of Marine Sciences at
The Maritime Studies Program of
Williams College and Mystic Seaport
____________________________________
Date
FUNDING
This work was supported by grants from the Center for Marine Biology, the
Department of Zoology, and the Graduate School of the University of New Hampshire.
i
ACKNOWLEDGEMENTS
Thank you, Dr. Larry Harris for all of your continued support, guidance, and
generosity over the past few years. I will always remember your encouragement to keep
exploring, observing, and writing everything down. These are valuable lessons that will
stay with me in all of my future studies. Thank you, Dr. Tom Lee for your inspiration in
community ecology class and also outside of class. Your enthusiasm for both the history
and present applications of ecology is contagious. Thank you, Dr. Jeb Byers for your
constructive comments and insights on this work, especially with an eye for the analyses.
Working with you has really taught me the value of taking broad-scale ecological
concepts and applying them to my research in a meaningful way. Thank you, Dr. Jim
Carlton; you inspired me to shift my path in 2000 and begin my exploration of science as
a career. I have learned that taking a holistic approach to research and teaching will yield
more meaningful results. I truly appreciate your unceasing support, kindness, and
generosity.
Thank you, graduate students from the Harris and Byers labs: Irit Altman, April
Blakeslee, Jenn Dijkstra, Aaren Freeman, Blaine Griffen, Wan Jean Lee, John Meyer,
Laura Page, and Erica Westerman for all of your detailed feedback throughout the
multiple stages of my research. Thank you to the following undergraduate assistants who
volunteered in the lab and in the field, creating their own projects and assisting with
aspects of my research: Jillian Armstrong, Scott Conley, Stephanie Doyle, Brian
ii
Donohue, Jilli Dur, Chris Hunt, Kathryn Lanza, Emily Strauss, Jessica Ware, Carolyn
Gordon, and Anthony Dutton.
Thanks go out to faculty, staff, and students from the Maritime Studies Program of
Williams College-Mystic Seaport in Mystic, CT. For assistance in the field, thank you
Rachel Fineman for your positive attitude and bright spirit even before the sun rises.
Thank you for assistance in providing housing, supplies, and a welcome home away from
home: Michelle Armsby, Nicole Dobrowski, Deniz Haydar, Mary O'Loughlin, and
Amanda Severin.
Thank you, Drs. Becky Warner, Jana Davis, Lara Gengarelly, Tim Miller, Pablo
Munguia, Michelle Scott, and Catherine deRivera for assistance on various matters
including statistical analyses and presentation of my thesis-work. Thank you, Dr. Andy
Rosenberg for motivating me to contemplate broad ecological questions that have
conservation implications, which I will carry with me to my future career. Thank you, Dr.
Steve Hale for introducing me to the art of teaching the ecology lab during my first year
at UNH.
Thank you, fellow researchers and staff at the Coastal Marine Lab at the
University of New Hampshire in Newcastle, NH including Noel Carlson and Nate
Rennels for your technical assistance. Thank you to Nancy Wallingford and Diane
Lavalliere for your assistance with many essential details pertaining to being a graduate
student in the Department of Zoology, and thanks to Dr. Jim Haney for providing
additional assistance especially with regard to my thesis defense.
Thank you to my friends who have not only spent countless hours talking about
science but also have been a network of support for me during the times when I needed it
iii
most: Irit Altman, Michelle Chalfoun, Amy Cline, Ben Galuardi, Deniz Haydar, Tami
Huber, Kate Magness, Chris Rillahan, Lynn Rutter, Amy Wolf, and Erika Zollett. Thank
you to my family Patricia Teck, Julio Lorda Solórzano, Rose Marie and Eijk Van
Otterloo, and Myrna Teck and Loren Amdursky for all of your love and kindness.
I am grateful to my parents Bruce and Susan Teck; your spirits will always live inside
me encouraging me to persevere and follow my dreams.
iv
TABLE OF CONTENTS
FUNDING………..………………………………………………………………………iii
ACKNOWLEDGEMENTS................................................................................................iv
LIST OF TABLES..............................................................................................................xi
LIST OF FIGURES..........................................................................................................xiii
ABSTRACT.......................................................................................................................xv
CHAPTER
PAGE
GENERAL INTRODUCTION............................................................................................1
Predator-Prey Interactions………………………………………………….…………1
Unsuccessful Predation…………………………………………………….………….3
Variability in Shell Form………………………………………………….…………..4
Littorina saxatilis Variability in the Northeast Atlantic………………………….…...6
Predators of Littorina saxatilis in the Northwest Atlantic………………………….…8
Littorina saxatilis Variability in the Northwest Atlantic…………………………….10
I. VARIATION IN LITTORINA SAXATILIS MORPHOLOGY ACROSS NEW
ENGLAND: SHELL THICKNESS AND EVIDENCE OF CRAB PREDATION
ATTEMPTS
Abstract........................................................................................................................13
Introduction..................................................................................................................14
v
History of Non-native Crabs in New England…………………………………...17
Objectives..............................................................................................................19
Methods .......................................................................................................................20
Field Sites..............................................................................................................20
Sample Sizes and Height Ranges for Examining Thickness of
Unscarred Snails....................................................................................................21
Snail Shell Thickness and Scarring Frequency……………………………..........22
Crab Density and Biomass………………………………………………….........23
Shell Thickness of Unscarred versus Scarred Snails…………………………….24
Analyses …………………………………………………………………….…...24
Results………………………………………………………………………………..25
Shell Thickness and Height Differences across Regions………………………...25
Crab Density and Biomass across Regions………………………………………26
Shell Thickness and Scarring Frequency………………………………………...27
Shell Thickness of Unscarred versus Scarred Snails…………………………….28
Discussion……………………………………………………………………………28
II. VARIATION IN LITTORINA SAXATILIS MORPHOLOGY ACROSS
THE GULF OF MAINE: SNAIL VULNERABILITY TO CRAB PREDATION
Abstract…………………………………………………………..…………………..49
Introduction…………………………………………………………………………..49
Objectives…….………………………………………………………………….51
Methods………………………………………………………………………………51
Snail Morphology……………………………………………………………..…51
vi
Snail Vulnerability to Predation………………………………………………….53
Results…………………………………………………..……………………………54
Snail Morphology……………………………………..…………………………54
Snail Vulnerability to Predation……………………….…………………………55
Discussion……………………………………………………………………………55
III. DOES SUB-LETHAL INJURY PROVIDE PREY WITH A REDUCED RISK OF
LETHAL PREDATION?
Abstract…………………...………………..……………...…………………………67
Introduction………………………………...…………..………….…………………67
Objectives……………………………………..…………………………………69
Methods………………………………………………………………………………69
Results……………………………………………………………………..…………71
Discussion……………………………………………………………………………71
SUMMARY.......................................................................................................................76
Future Directions……………………………………………………..……………...79
LITERATURE CITED……………………………………………………..……………81
APPENDICES………………………...…………………………………………………91
APPENDIX A: Field Site Details..……….…………………………….………..…..92
APPENDIX B: Shell Thickness Comparisons between Field Sites within each
Region…………………………………………..………………………………..…..96
vii
APPENDIX C: Additional Results Comparing Crab Predation of Snails Differing in
Thickness……………...…………..…………..…………………………………....100
APPENDIX D: Comparison of Odiornes Point versus Wilbur Neck Snail
Movement……..…….………………………………………………………….…..101
APPENDIX E: Comparing Claw Sizes between Hemigrapsus sanguineus and
Carcinus maenas……………………………………………………………..……..102
APPENDIX F: Handling Time Details for Trials with Carcinus maenas and
Hemigrapsus sanguineus Preying upon Scarred and Unscarred Snails……..….…..103
viii
LIST OF TABLES
TABLE
PAGE
Table 1.1 Number of snails measured per size class per region………….…………….34
Table 1.2a Summary of each site: unscarred snails 5-13 mm in height: shell thickness,
shell height, and number of snails measured…………………………………………….34
Table 1.2b Summary of each region: unscarred snails 5-13 mm in height: mean shell
thickness, mean shell height, and number of sites in each region…………...…………..35
Table 1.3a Summary of each site: mean crab density, % C. maenas, % H. sanguineus,
and number of quadrats………………………...…………………………………...……35
Table 1.3b Summary of each region: mean crab density, % C. maenas, % H.
sanguineus, and number of quadrats..………………………………………….………...35
Table 1.4a Summary of each site: mean total crab biomass, mean biomass of C. maenas,
mean biomass of H. sanguineus, and number of quadrats.……………………........……36
Table 1.4b Summary of each region: mean total crab biomass, mean biomass of C.
maenas, mean biomass of H. sanguineus, and number of quadrats…………..…………36
Table 1.5a Summary of each site: number of snails collected, number of scarred snails
collected, and mean scarring frequency, number of samples collected per site, and percent
of total snails collected blemished per site.…………………………………….……......37
Table 1.5b Summary of each region: number of unscarred snails collected, number of
scarred snails collected, mean scarring frequency, number of sites in each region, and
average percent of total snails collected blemished per region.………………….………37
Table 1.6 Mean scarring frequencies at the five most heavily scarred sites within the
three southern regions.…………………………………………………...………………38
Table 1.7 Number of snails measured per size class per region for scarred and unscarred
snails……………………………………………………….……………………….……38
Table 2.1a ANCOVAs for snail dry tissue and shell weight with shell height as the
covariate…………………………………………………………………………...……..60
ix
Table 2.1b Differences in L. saxatilis dry tissue and shell weight between the sites..…60
Table 2.2a ANCOVAs for shell width, thickness and spire height with shell height as
the covariate………………..……………………………………………………...……..61
Table 2.2b Differences in L. saxatilis shell width, thickness and spire height between the
sites.…………………………………………………………………………….………..61
Table 3.1 Scarred versus unscarred snails’ handling times for C. maenas and H.
sanguineus………………………………………………………………………………..75
Table A.1 Site names and codes…………………...……………………………………92
Table A.2 Individual sites’ coordinates…………………………...…………………….93
Table A.3 Individual sites’ mean shell height and thickness across all sampled
snails…………………………………………………………………………………......93
Table A.4 Individual sites differences in mean crab density……………...……………94
Table F.1a Outcomes of 124 handling time trials for C. maenas and H. sanguineus
preying upon scarred and unscarred snails matched for height…………………….…..103
Table F.1b Choice results for trials with C. maenas and H. sanguineus during which
both unscarred or scarred snails were handled…………………………….………...…104
Table F.2 Relationships between handling time and crab to snail size ratio for unscarred
and scarred snails handled by both C. maenas and H. sanguineus…………………..…104
x
LIST OF FIGURES
FIGURE
PAGE
Figure 1.1 Map of four regions where sampling occurred…………………...…….…...39
Figure 1.2 Scars as evidence of previous crab predation attempts on L. saxatilis
shells……………………………………………………………………………………..40
Figure 1.3a Aperture thickness against shell height for the four regions….…………...41
Figure 1.3b Aperture thickness against snails of particular size classes of shell height for
the four regions…………………………………………………………………………..42
Figure 1.4a Mean snail shell thickness at the aperture against latitude………………...42
Figure 1.4b Mean snail shell thickness at the aperture against temperature….………...43
Figure 1.5 Mean shell thickness at the aperture against mean height………….……….43
Figure 1.6 Mean scarring frequency against mean crab density………………………..44
Figure 1.7 Mean shell thickness at the aperture against mean crab density……….…...44
Figure 1.8 Mean crab density across sites within the four regions………………….….45
Figure 1.9 Mean crab biomass across sites within the four regions…….……………...45
Figure 1.10 Mean scarring frequency against mean shell thickness at the aperture…....46
Figure 1.11 Mean scarring frequency against latitude…….……………………………46
Figure 1.12 Aperture thickness for unscarred and scarred snails…….………………...47
Figure 1.13 Aperture thickness of snails with scars close (0.5-1.5 mm) to the aperture
edge, with scars far (1.5-3.5 mm) from the aperture edge, and without scars are
compared…………………………………………………………………………………48
Figure 2.1 Map of the field sites: Wilbur Neck in northeastern Maine (WN) and
Odiornes Point, New Hampshire (OP)..………………………………….………………62
xi
Figure 2.2 Snail dimension measurements diagram…………………….……………...63
Figure 2.3 Dry tissue weight against shell height…………………….………………...64
Figure 2.4 Dry shell weight against shell height…………….………………………….64
Figure 2.5 Shell width against shell height…….……………………………………….65
Figure 2.6 Shell thickness at the aperture against shell height……….………………...65
Figure 2.7 Spire height against total shell height…………………………….…………66
Figure 2.8 Photo of snails matched for length (6.85 mm ±0.05) from Odiornes Point,
New Hampshire (OP) and Wilbur Neck, Maine (WN)…………………………………..66
Figure 3.1 Carcinus maenas and Hemigrapsus sanguineus handling times for scarred
and unscarred L. saxatilis……………………..………………………………………….75
Figure A.1 Average monthly temperatures for four sites closest to the four regions
examined………………………………………………………………………………....95
Figure B.1 Comparison of sites within the Southernmost region (shell thickness at the
aperture against shell height)……………………………………………….……………96
Figure B.2 Comparison of sites within the Southern region (shell thickness at the
aperture against shell height)………………………….…………………………………97
Figure B.3 Comparison of sites within the Northern region (shell thickness at the
aperture against shell height)………………………….…………………………………98
Figure B.4 Comparison of sites within the Northernmost region (shell thickness at the
aperture against shell height)……………………………….…………………………....99
Figure D.1 Comparison of snail movement from Northeastern Maine and New
Hampshire……………………………………………………………………………....101
Figure E.1 Diagram of claw measured to calculate claw area…………………….......102
Figure E.2 Comparing claw sizes between Hemigrapsus sanguineus and Carcinus
maenas…………………….............................................................................................102
Figure F.1 Carcinus maenas and Hemigrapsus sanguineus snail handling time against
crab to snail size ratio…………………………………...……………………..……….105
Figure F.2 Snail heights of consumed snails against the carapace widths of the Carcinus
maenas and Hemigrapsus sanguineus that ate the snails…………………………….....106
xii
ABSTRACT
NATIVE GASTROPODS AND INTRODUCED CRABS: SHELL MORPHOLOGY
AND RESISTANCE TO PREDATION IN THE NEW ENGLAND ROCKY
INTERTIDAL ZONE
by
Sarah J. Teck
University of New Hampshire, December, 2006
The impact of non-native species is one of the most critical issues facing management
and conservation today. When these invaders are generalist predators, their impacts on
native communities can be a major restructuring force for ecosystems. A new voracious
predator the Asian crab, Hemigrapsus sanguineus, joins another non-native established
species, Carcinus maenas, along the majority of the New England coastline. What
remains poorly understood is how the two introduced predators may modify local
communities, especially considering their impact on native prey, such as the rough
periwinkle snail, Littorina saxatilis. The goal of this research is to investigate the
vulnerability of these snails to shell-breaking predators by examining their clinal
variation in shell morphology and crab-induced scarring history. Focused studies on the
variable morphology of this native gastropod coupled with predation studies provide a
greater understanding of the ecological and evolutionary consequences of the arrival of
novel predators to an ecosystem.
xiii
GENERAL INTRODUCTION
Introduced species are the second greatest cause of human induced ecological change
(second only to alteration and destruction of habitat) (Park 2004). When these introduced
species are omnivorous predators, understanding their impact on native organisms
becomes fundamental to understanding how introduced species restructure ecosystems
(Elton 1958, Vitousek et al. 1996). Around the world, introduced crabs have impacted
recipient environments by altering community structure and decimating native prey
populations (Glude 1955, Dare et al. 1983, Carlton and Geller 1993, Lodge 1993,
Grosholz and Ruiz 1996, Ruiz et al. 1998, Lohrer and Whitlatch 2002a). It is critical to
consider the role of individual species within a predator-prey network and especially how
an exotic species influences native prey (Lodge 1993). Understanding the impact of
specific introduced predatory species that may cause subtle shifts in trophic interactions
is crucial as communities become increasingly rich with non-native species.
Predator-Prey Interactions
Predators must contend with a prey item’s defenses in order to successfully and
efficiently consume a prey. Prey develop these defenses through optimal physical or
behavioral adaptive mechanisms to escape predation encounters (Vermeij 1987); prey
avoid predators to decrease the likelihood of encountering them and/or develop armor to
reduce predator efficiency and success in handling them (Seitz et al. 2001). Adaptation
occurs through selection for constitutive defenses (which are always present) (Vermeij
1
and Currey 1980, Vermeij 1982b, Seeley 1986), inducible defenses (which are activated
with a stimulus) (Appleton and Palmer 1988, Etter 1988, Harvell 1990, Trussell 1996,
Leonard et al. 1999, Dalziel and Boulding 2005) or a combination of the two
(Johannesson and Johannesson 1996). Prey items must not only contend with the risk of
predation but also other biotic factors, such as competition, and abiotic stresses, such as
wave exposure, in their environment. Species must shift in morphology and behavior as a
result of a spectrum of stresses each individual faces in its lifetime or in one or several
generations of a population. Since predation success is dependent on morphological and
behavioral traits of prey, investigation into these traits will be indicative of an organism’s
potential vulnerability to predation.
Predators seek to consume the most energy while expending as little energy as
possible in pursuing, handling, and consuming prey, as optimal foraging theory states
(MacArthur and Pianka 1966, Bowen et al. 2002). However, prey specifically develop
characteristics to interfere with the process of predators finding, subduing/handling, and
consuming prey (Hughes and Elner 1979) through avoidance, armor, and chemical
deterrents, respectively, and any reduced predator efficiency is associated with an
increased risk of unsuccessful predation (Nilsson and Bronmark 2000, Greenfield et al.
2002). Prey that are large (Floyd and Williams 2004), exhibit plastic phenotypic traits, or
characteristics that otherwise protect the prey may be selected for due to the low
efficiency in predator handling time (Ebling et al. 1964, Vermeij 1982d, Palmer 1985,
Seeley 1986, Appleton and Palmer 1988, Schindler et al. 1994, Kolar and Wahl 1998) or
high risk of predator injury (i.e. claw damage to crabs) (Smallegange and Van der Meer
2003), competition, and predation on the predator itself (Brante and Hughes 2001). Here I
2
will examine morphological features of shelled prey that have developed under the
pressure of shell-breaking predators.
Unsuccessful Predation
Unsuccessful predation can be recorded historically in the form of shell scars on
many gastropod species (Vermeij 1982d) and is an indicator of the strength of selection
present in the system (Vermeij 1982b). Failed predation attempts suggest that prey is
evolving to reduce shell-breaking predator success. The rate of phenotypic evolution
increases with decreasing latitude (Vermeij 1982d), and is evidenced by the increased
frequency of repairs in lower latitudes (Vermeij et al. 1980). Shells tend to be thicker in
lower latitudes in general due to an increased ability to accrete shell in warmer waters
(Vermeij and Currey 1980) and to predator chemical cues inducing phenotypic defenses
(Trussell and Etter 2001). Thus, scarring also increases both with increased temperatures
and predator abundance and diversity (Vermeij 1978).
These non-lethal encounters resulting in chipped shells heal to form scars, which tend
to thicken the shell decreasing the likelihood for future successful predation (Greenfield
et al. 2002). Although there is apparently no difference in the overall strength of the
shells (Blundon and Vermeij 1983, Greenfield et al. 2002), scarred shells are thicker than
unscarred snails at the aperture. Snails do not thicken their entire shell when they are
healing from a previous crab attack because the energy required to repair a shell is likely
associated with a reduction in energy allocated for growth and reproduction since
accreting new shell is energetically expensive (Geller 1990b, Greenfield et al. 2002).
However, energy allocation for shell repair is not consistent across species. In
3
experimental trials, the freshwater snail, Helisoma trivolvis, increased growth when
damaged with no increase in fecundity and mortality (Stahl and Lodge 1990). Shelldamaged Nucella emarginata in California also had increased shell growth, but they had
increased mortality and egg production when compared to uninjured snails (Geller
1990b).
Variability in Shell Form
Shell forming mollusks often have been examined because they reflect ecological and
evolutionary changes over time and space (Vermeij 1987). Species that have limited
genetic mixing based on their reproductive cycle and low rate of widespread adult
dispersal often have highly variable phenotypes among separate populations as the result
of local adaptation. The poorly dispersing snail, Nucella lapillus, displayed rapid
evolution by increasing shell thickness in response to the arrival of a new predator,
Carcinus maenas, in comparison to the widespread-spawning snail, Littorina littorea,
which did not change in shell thickness (Vermeij 1982c). Nucella lapillus also has
displayed clear differences in shell thickness across a landscape; individuals found on
sheltered shores have thicker shells than those on more exposed shores in North Wales
(Ebling et al. 1964, Hughes and Elner 1979) and in Spain (Rolan et al. 2004). Individuals
at protected sites have more time to feed and thus can spend more energy thickening their
shells in order to resist the predators that are more abundant here than at exposed sites.
Additionally, N. lapillus have a larger aperture and pedal surface area to resist high wave
exposure in such sites as opposed to those on more protected shores in North Wales
(Ebling et al. 1964) and in New England (Etter 1988). A smaller aperture for snails in
4
sheltered areas is also more beneficial to reduce the success of predators peeling the shell,
and such predators are more abundant in sheltered areas. The closely related Nucella
lamellosa on the west coast of Canada has been shown to develop larger teeth along the
aperture when exposed to chemical cues of the predatory crab Cancer productus
(Appleton and Palmer 1988). Chemical cues from the predator Hemigrapsus nudus (also
on the west coast of Canada) induces Littorina subrotundata to produce a more massive
shell for greater protection (Dalziel and Boulding 2005). Another predatory crab,
Carcinus maenas, has influenced shell shape in Littorina obtusata in New England;
before 1900 snails were thinner with higher spires than those collected between 1982 and
1984 when the non-native crab was more abundant (Seeley 1986). However, Trussell
(1996) attributes this morphological difference to a combination of rapid morphological
evolution explained by Seeley (1986) and phenotypic plasticity induced by chemical cues
from C. maenas. Additionally, Trussell and Etter (2001) discovered that L. obtusata from
the northern Gulf of Maine displayed a greater capacity for phenotypic plasticity than
conspecifics in the southern Gulf of Maine when transplanted into warmer water,
suggesting that these two populations are genetically disparate (northern snails increased
in shell thickness by 43% in southern waters versus local northern snails, while southern
snails in northern waters had thinner shells by 18% versus local southern snails). There
might be a greater selection for rapid growth and efficient shell accretion in the north,
where water temperatures are much colder. Across littorinid species in the Northeastern
Pacific, there is also selection for thicker shells, and the thinner-shelled species, Littorina
subrotundata, suffers from significantly greater predation by crabs than the thicker-
5
shelled species, L. sitkana and L. scutulata (Boulding and Van Alstyne 1993, Boulding et
al. 1999).
Littorina saxatilis Variability in the Northeast Atlantic
A species which exhibits a high level of variability in morphology is the rough
periwinkle, Littorina saxatilis, which broods its young. Littorina saxatilis occurs from
New Jersey to Hudson Bay, Baffin Island, the MacKenzie Delta, Greenland and the
Barents Sea and in Europe from Gibraltar to Novaya Zemlya (a Russian island in the
Arctic Ocean). This wide range may have resulted from the passive transport of L.
saxatilis on drifting seaweed over evolutionary time (Carlton, J.T., personal
communication, Johannesson 1988), since this species typically has low dispersal of their
crawl-away young.
Littorina saxatilis in Europe are well recognized as displaying high variability in shell
shape across tidal ranges, habitats, and latitudes due to differing levels of predation
pressure, wave exposure, food availability and temperature (which in turn affects the rate
of shell accretion) (Grahame et al. 1990, Clarke et al. 1999, Leonard et al. 1999).
Individuals of L. saxatilis have been shown to differ genetically from conspecifics only
several meters away (Janson and Sundberg 1983, Johannesson 2003, Grahame et al.
2006). Foot size in L. saxatilis enlarges with increasing exposure for snails found in the
United Kingdom (Grahame and Mill 1986), and this also occurs as a plastic phenotypic
response in congener L. obtusata (Trussell 1997) and Nucella lapillus in New England
(Etter 1988). Also in the U.K., two ecotypes, found in the high (H) and mid (M) intertidal
zones (Reid 1996, Wilding et al. 2002), have been shown to exhibit assortative mating
(Pickles and Grahame 1999) and genetic differentiation (Wilding et al. 1998, Grahame et
6
al. 2006). Littorina saxatilis ‘H’ (once described as L. patula (Wilding et al. 2002)) has a
thin shell and a large aperture and is found in the high shore among cliffs and boulders,
and L. saxatilis ‘M’ (once described as L. rudis) has a thick shell and a small aperture and
is found in the mid-shore among small boulders (Wilding et al. 1998, Hull et al. 1999).
Similar patterns in shell shape and strength are found in N. lapillus in Ireland (Ebling et
al. 1964). In addition to small scale variation in shell shape, L. saxatilis exhibits clinal
variation across the U.K.; for example, snails increase in aperture size from Northern
England to the South and then to the East from Devon and increase in jugosity (ratio of
aperture length to columella length) from Cornwall to the North to the Isle of Man and to
the East to Kent (Mill and Grahame 1995).
In Sweden two additional ecotypes are described; small snails with thin shells and
large apertures are found in exposed cliff sites (the E morph), and larger snails with
thicker shells and smaller apertures are found in sheltered boulder areas (the S morph)
with intermediate forms found in between (the I morph) (Janson and Sundberg 1983,
Johannesson 1986, Johannesson and Johannesson 1996, Hollander 2001). Differences in
shell form and growth rates between these ecotypes are likely influenced both by genetic
and environmental variation (Janson 1982). Snails found on sheltered shores grow faster
than those from exposed areas (Janson 1982) likely due to predation pressure by crabs
(Raffaelli and Hughes 1978, Elner and Raffaelli 1980) and the increased feeding time,
and females often grow at a faster rate than males in several other congeners
(Johannesson and Johannesson 1996). Littorina obtusata in New England also differ with
varying exposure regimes; snails found in exposed sites with fewer predators have
7
thinner shells (Trussell 1996) and larger feet (Trussell 1997) than snails found in
protected areas.
In Galicia, Spain, two morphs of L. saxatilis are found in different intertidal habitats,
the RB-morph is ridged and banded and found in the upper tidal heights among barnacles
and the SU-morph is smooth and unbanded and found in the lower tidal heights among
mussels (Kostylev et al. 1997, Carballo et al. 2001). The RB morph is also comparable to
the sheltered morph in Sweden, as its shell is globular and robust with a smaller aperture
than the SU morph. These traits offer protection against crabs which are also more
common in the upper portion of the intertidal with the RB morph (Cruz et al. 2004,
Carvajal-Rodriguez et al. 2005). The genetic and plastic morphological variation in this
species has been closely examined in its European range, but there are limited studies of
this species in its Northwest Atlantic range (Bertness 1999).
Predators of Littorina saxatilis in the Northwest Atlantic
A likely selective force in the Northwest Atlantic on L. saxatilis is the introduced
European green crab, Carcinus maenas, which has overlapped with L. saxatilis, since its
arrival on the New England shores in the early 1800s; however, these two species have a
long evolutionary history in Europe. Carcinus maenas was introduced to New England
prior to 1817, and by the mid-1900s its abundance increased along the shores of northern
New England and Canada (Glude 1955). A combination of warming trends (Scattergood
1952, Welch 1968, Welch and Churchill 1983, Audet et al. 2003) and pulses of new
invasion events (Roman 2006) explain the range expansion of C. maenas in New England
and Eastern Canada. In high numbers C. maenas can have significant impacts upon prey
8
populations, particularly mollusks (Glude 1955, Dare et al. 1983, Lohrer and Whitlatch
2002b, Floyd and Williams 2004). Carcinus maenas also resides among juvenile lobsters
and has the potential to compete with and prey upon this commercially valuable species
(Rossong et al. 2006, Williams et al. 2006).
The more recent introduction of Hemigrapsus sanguineus to the Atlantic coast may
have an even greater impact upon invertebrate prey populations than C. maenas (Tyrrell
and Harris 1999, Lohrer and Whitlatch 2002b, Brousseau and Baglivo 2005). However,
interspecific crab predation and competition may influence the impact on their shared
prey items. Some prey items may actually have a reduced risk of predation when these
two predators interact (Griffen and Byers 2006a), and results likely are influenced by
habitat type (i.e. rock versus macroalgae) (Griffen and Byers 2006b).
About five years after the discovery of H. sanguineus in North America, it arrived in
southern New England, where populations dramatically increased, supplanting the longestablished European green crab invader, Carcinus maenas (McDermott 1998, Ahl and
Moss 1999, Tyrrell and Harris 1999, Lohrer and Whitlatch 2002a). Hemigrapsus
sanguineus was first found in New Jersey in 1988 (Williams and McDermott 1990), and
since its discovery, it has been spreading north, extending its range into Maine
(McDermott 1998, Tyrrell and Harris 1999). Hemigrapsus sanguineus now surpasses C.
maenas in density in many intertidal locations (McDermott 1998, Ahl and Moss 1999,
Lohrer and Whitlatch 2002a). The multiple genetic lineages of C. maenas in New
England have allowed this species to pervade in locations once thought to be too cold for
C. maenas (Roman 2006), so H. sanguineus is likely to expand its range further north and
increase in dominance, especially if new invasion events occur and as waters warm.
9
However, Byers and Pringle (2006) predict that populations of H. sanguineus in midMaine and further north will remain ephemeral unless water temperatures do in fact
increase.
In the Western Atlantic, there may be different predator-prey dynamics on the
northern (eastern) versus southern (western) sides of Cape Cod due to temperature and
predation history. Temperature differences influence growth, feeding, and healing rates
of snails (Vermeij and Currey 1980), and increased temperatures south of Cape Cod
increase crab residency in the intertidal resulting in greater exposure of prey to predators.
Historically, Cape Cod may have been a barrier to the rapid spread of C. maenas—south
of Cape Cod C. maenas has been present for ~200 years, whereas north of Cape Cod it
has been present for ~100 years (since 1905) (Roman 2006). In contrast, H. sanguineus
very quickly spread from south to north and within two decades reached areas well north
of Cape Cod (perhaps the opening of the Cape Cod Canal in 1914 increased the speed of
range expansion of this later invasion).
Littorina saxatilis Variability in the Northwest Atlantic
Snail characteristics vary with latitude due to differing crab abundance, wave
exposure, and temperature regimes (Boulding and Van Alstyne 1993, Boulding et al.
1999). Additionally, Littorina obtusata have been shown to exhibit phenotypic plasticity
in shell shape likely due to a combination of differences in crab abundance and water
temperature. Northern New England snails are much thinner and weaker than southern
New England snails (Trussell 2000), which is a trend across many species of shell
bearing mollusks (Vermeij 1978, Vermeij and Currey 1980, Irie and Iwasa 2003).
10
Carcinus maenas also have smaller crusher claws in the northern versus the southern
Gulf of Maine, likely due to a reduced need for large claws to crush the generally thinner
snails (Smith 2004). In the short time period H. sanguineus has been present in southern
New England, the mussel Mytilus edulis showed rapid evolution of inducible defenses
when exposed to chemical cues from this new predator, while mussels further north did
not (where H. sanguineus is not established yet); both populations responded to the older
invader, C. maenas (Freeman and Byers 2006). Thus, other prey are likely to exhibit
similar species-specific responses to predators, and these responses can rapidly evolve
based on the intensity and composition of predators present in the system.
Both prey and predator characteristics have varied in the past and likely will vary in
the future with different predator species regimes, ranging from the 1800-1995 single
invasive crab species regime (C. maenas), to today with two non-native crab species
present (C. maenas and H. sanguineus). Whether H. sanguineus causes C. maenas
populations to increase, decrease, or remain unchanged, there is the potential in all
scenarios for total crab abundances in the intertidal areas to increase. While this new
invader may not in fact cause total crab abundance in the intertidal rocky shores to
increase, it may redistribute crab abundances in areas that impact a different tidal height
more heavily and thus a different assemblage of species. Researchers have a poor
understanding of how the presence of these two invasive species together will affect
native coastal species and how they may each have a unique influence on their prey.
Recent studies have focused on the habitat and prey selection of these invasive crabs
(Tyrrell 1999, Lohrer et al. 2000a, Lohrer et al. 2000b, Brousseau et al. 2001, Ledesma
and O'Connor 2001, Tyrrell and Harris 1999, Tyrrell 2002, Bourdeau and O'Connor
11
2003), but there is a paucity of research that focuses specifically on the morphology of
native prey species in New England and their specific vulnerability to these predators
across their range (but see Vermeij 1982c, Seeley 1986, Trussell 1996, Trussell and
Smith 2000, Freeman and Byers 2006). No current research specifically addresses the
consequences of both crab invaders, H. sanguineus and C. maenas, on herbivores, such as
L. saxatilis, which have an vital role in regulating algal populations in intertidal
communities. Additionally, L. saxatilis has not been studied extensively in its North
American range (Bertness 1999), so the following studies will provide essential baseline
information on the morphology and vulnerability of this important and abundant native
intertidal species.
In Chapter I, I examined the clinal variation of Littorina saxatilis across its New
England range. I hypothesized that snails found in the northern portion of New England
exhibit thinner shells than those found further south. Unscarred snails are also thinner
than crab-scarred snails as expected, and there is a greater frequency of scarred snails
when their shells are thicker. I also examined the frequency of crab-scarred snails and
crab abundance across New England. As was documented in Chapter II, in general
thinner-shelled snails in the north will likely be more vulnerable to predation by crabs
than thicker shelled conspecifics in the south. In Chapter III, I tested my prediction that
scarred snails have a decreased risk of future predation due to their likely thicker shell
where the previous chip has healed.
12
CHAPTER I
VARIATION IN LITTORINA SAXATILIS MORPHOLOGY ACROSS NEW
ENGLAND: SHELL THICKNESS AND EVIDENCE OF CRAB PREDATION
ATTEMPTS
Abstract
In the Northwestern Atlantic, the morphology of the rough periwinkle, Littorina
saxatilis, has not been studied in such detail as its European populations. Clinal variation
in shell thickness exists across its New England range; snails found in the northern
portion of New England exhibit thinner shells than those found further south. I also
examined the frequency of crab-scarred snails and crab density in New England. Scarring
thickens the shell, and thus leaves a snail less vulnerable to future predation. Examining
scarring frequency may indicate the degree of selection for thick shells present at a
particular site. Previous studies indicated that crabs were rare in northern New England
and Canada and thus have not presented a substantial threat to local snail populations.
More recent studies have shown that crabs are becoming more common in the northern
reaches of their ranges, and thus northern snails may now be more vulnerable to crab
predation.
13
Introduction
The morphology of the common rocky shore rough periwinkle, Littorina saxatilis,
has been studied extensively in Europe, but in less detail elsewhere across its North
Atlantic range in Iceland, Greenland, and North America. Populations of L. saxatilis
show high variation in shell morphology across spatial scales. Two ecotypes of L.
saxatilis have been examined in British populations: L. saxatilis ‘H’ (with thin shells and
large apertures found in high shore among cliffs and boulders) and L. saxatilis ‘M’ (with
thick shells and small apertures found in mid-shores among smaller boulders) (Wilding et
al. 1998, Hull et al. 1999). L. saxatilis ‘M’ probably corresponds to what was once
described as L. rudis (Wilding et al. 1998), and L. saxatilis ‘H’ was often referred to as L.
patula (Wilding et al. 2002). Both are now regarded as ecotypes of L. saxatilis (Reid
1996, Wilding et al. 2002). In Swedish populations of L. saxatilis, two ecotypes have
been identified as the E-morph, which have thin shells and a large foot/aperture (found in
exposed areas), and the thicker-shelled, small apertured S-morph (found in sheltered
areas) (Janson and Sundberg 1983, Johannesson 1986, Johannesson and Johannesson
1996, Hollander 2001).
Since there are fewer studies on L. saxatilis in its northwest Atlantic range, I focused
on large scale clinal variations in L. saxatilis across latitude rather than small scale
variations across exposure and tidal height/substrate. Since it is easier to accrete calcium
carbonate shell at higher temperatures (Vermeij and Currey 1980), snails in lower
latitudes are typically thicker than those in higher latitudes. Often there is a greater
predation pressure in these lower latitudes as well (a greater abundance and species
diversity of crabs), so there is a greater selective advantage for thicker shells (Vermeij
14
and Currey 1980, Vermeij 1982d, Vermeij 1987). Thicker shelled snails are not as
reproductively productive as thinner shelled snails, however snails will upkeep their
shells in terms of growth and repair equally (Geller 1990a).
Snail shell thickness will indicate a species’ potential vulnerability to shell-breaking
predators. Crabs often “peel” the shell by chipping away at the aperture until the tissue of
the snail is reachable (Bertness and Cunningham 1981, Vermeij 1982b, Lindstrom 2005);
crabs usually only crush the snail if there is very large crab to snail size ratio (Bertness
and Cunningham 1981). Crabs prefer to break thinner versus thicker shelled snails (Elner
and Raffaelli 1980, Palmer 1985, Geller 1990a). A snail may be left chipped and uneaten
if a crab cannot quickly access the snail tissue, so there is typically a greater proportion of
scarred snails that are thick-shelled than thin-shelled (Elner and Raffaelli 1980, Vermeij
1982a). This chipped snail will form a scar as the shell re-grows. New shell grows
beneath the outer edge of the break (Lindstrom 2005), thus it is usually thicker than the
surrounding unscarred shell and leaves a noticeable ridge revealing the shape of the
original chip. General thickening of the shell may occur, based on the amount of
exposure the snail has to crab chemical cues. However, this ridge is likely the
physiological byproduct of the shell repairing process and not likely induced as a
phenotypic response to the crab encounter.
Crab-induced scars can be distinguished from those caused by abiotic forces because
they are repeated in a distinctly regular pattern (easily observed both in field collections
and lab predation experiments). Abiotic scars, caused by wave action knocking shells off
rocks or rocks being turned over, result in chips and cracks that are random and highly
variable in nature, and these scars are not as common as those caused by lip-peeling
15
predators (Cadee et al. 1997, Lindstrom 2005). Additionally, Lindstrom (2005) noted that
if abiotic forces were a major cause of shell injury there would be a higher frequency of
scarring in highly wave-exposed areas, which is not the case. Snails tend to have bigger
feet in more exposed areas, which also reduces the likelihood of abiotic-induced scarring
in these areas. Scarring generally increases in littorinids as latitude decreases (Vermeij
1982d), so scarring is likely positively related to shell thickness or higher predation.
Additionally, the accumulation of scars will increase as the snail gets older, and since L.
saxatilis can live up to 9 to 11 years, a snail can reflect the historical presence of shellbreaking predators in a particular location (Gorbushin and Levakin 1999).
The frequency of scarred snails in a population is an indication of the degree to which
selection for shell strength is present in the system (Vermeij 1982b), or how important
predators are in shaping the adaptations of their shelled prey. A high frequency of
scarring means that predators are likely a strong selective force in the system; snails have
likely evolved thicker shells due to predator’s presence, and this results in a greater
number of unsuccessful predation events, or scarred snail shells. A low number of scarred
snails in a population may be the result of at least two processes: shell-breaking predators
are not a substantial threat to the snails or shell-breaking predator attacks are primarily
lethal (Vermeij 1982a, Lindstrom 2005). Thus, it is difficult to gauge predator intensity
from solely examining crab-induced scars within a population. Additionally, some crabs
may leave no evidence of an attack, if they simply pull out the tissue of the snail from the
aperture without cracking the shell (Johannesson 1986). Although C. maenas is known to
be able to extract most of the animal of Littorina littorea out of its shell in the laboratory
(J. T. Carlton, personal communication), the species and size classes of crabs capable of
16
preying upon L. saxatilis primarily use the peeling technique to access snail tissue
(personal observations in the lab). Since scarred snails are typically thicker than
unscarred snails, crabs will likely take longer to handle these snails resulting in reduced
predation success. Additionally, snails that are simply thinner due to clinal differences in
shell thickness are likely more vulnerable to predation by shell-breaking crabs.
History of Non-native Crabs in New England
The history of the European green crab Carcinus maenas on the North American
Atlantic coast was reviewed by Carlton and Cohen (2003). From its appearance in the
early 1800s in Long Island Sound, it spread in the late 19th century to waters north of
Cape Cod, and by the mid-20th century had reached Nova Scotia. Surveys conducted in
1984, indicated that C. maenas was well-established in some areas of Northeastern Maine
and rare to absent in others (Seeley 1986). In this same area, there was a peak in crab
abundance in the late 1990s, which has now faded, so current populations are
inconsistently distributed among sites once again (Harris, L. G. and Robin H. Seeley,
personal communication, Matthews-Cascon 1997).
Fishermen in southern Nova Scotia (east of Northeastern Maine across the Bay of
Fundy) considered, C. maenas, to be abundant in 1964, but there seemed to be a
reduction in density over the subsequent years coinciding with reported colder than
average winters (Audet et al. 2003). Although the UK experienced a similar drop in crab
densities during this cooler period, populations of C. maenas rebounded faster than those
in its US exotic range. Not until the mid-1990s did Gulf of St. Lawrence, Canadian
fishermen begin to see that green crab abundance had increased once again and spread
17
into new areas (Audet et al. 2003). By 2000, shellfish farmers on Prince Edward Island
(even further North than Northeastern Maine) identified C. maenas as a major concern to
local aquaculture (Miron et al. 2005); on the north coast of Nova Scotia fishing gear was
reported to catch hundreds of green crabs within a 24-hour time period in 2003 (Audet et
al. 2003).
The 1990s range extension of C. maenas into the Gulf of St. Lawrence was initially
interpreted as a continued northward movement of the green crab from southern waters,
enhanced by warming coastal ocean temperatures (Audet et al., 2003). However, Roman
(2006) has shown, based upon genetic evidence, that these northernmost colder-water
populations represent a separate invasion of C. maenas from northern Europe. Although
there are limited data on present day population sizes and long-term decadal trends of C.
maenas in New England, as coastal ocean temperatures continue to rise, it is predicted
that green crabs will increase in density and range (Audet et al. 2003, Roman 2006). With
this likely expansion of green crabs, the spread of the newer non-native crab,
Hemigrapsus sanguineus is highly probable as well. However, this will likely happen
only with an increase in water temperatures (Byers and Pringle 2006).
The Asian shore crab, H. sanguineus, was first discovered in 1988 in New Jersey, and
has since spread both south to North Carolina and north to mid-Maine (McDermott
1998), reaching New Hampshire by 1998 (McDermott 1999). In southern portions of its
New England intertidal range, it reaches high densities and has largely replaced C.
maenas (Lohrer and Whitlatch 2002a). At the northern end of the range of H. sanguineus
in Maine, populations are still not as abundant as those of C. maenas. However, H.
sanguineus populations have increased considerably since 2001 when they were largely
18
absent from sites as far north as New Hampshire (Tyrrell 1999, 2002). Both crab species
overlap with L. saxatilis in New England’s intertidal rocky shores and are the most
frequent predators on these snails. Previous studies have assumed than northern New
England has a very low predation pressure from crabs due to low populations of C.
maenas coinciding with harsh northern New England winters. I propose here that crabs,
as reported most recently by Audet et al. in 2003, may begin to increase in number in the
northern reaches of its range and pose a threat to this snail species even in locations far
north which previously may have had only rare crab peaks of abundance (Seeley 1986,
Trussell and Smith 2000, Trussell and Etter 2001).
Objectives
Since shell thickness is one of the most important features determining a mollusk’s
susceptibility to predation by shell-breaking predators (Vermeij 1987) and scarring may
change shell thickness (Greenfield et al. 2002), I examined these features in Littorina
saxatilis at sites across New England. The measurements serve as baseline data on the
clinal variability in morphology of L. saxatilis in New England specifically in regard to
their vulnerability to crab predation. I hypothesize that snails in the North will have
thinner shells, indicating a greater degree of vulnerability to predation, than snails in the
south. Additionally, shell scarring will increase shell thickness, suggesting that scars
provide snails with a reduced risk of successful predation by shell-breaking predators.
19
Methods
Field Sites
To examine variation in the morphology of Littorina saxatilis, I clustered sampling
sites within limited regions to look at small scale differences and spaced these sampling
regions across a broad range to see larger scale differences. I chose four sampling regions
(with 2-5 sites within each region) ranging from northeastern Maine to Rhode Island
(Figure 1.1, Appendix A). These four regions will be referred to (from north to south) as
Northernmost (the area of Eastport, Maine), Northern (the area of Winter Harbor,
Maine), Southern (coastal New Hampshire/southern Maine), and Southernmost (the
Rhode Island/Connecticut border area). The Southernmost and Southern region are 213
km apart; the Southern and Northern regions are 259 km apart; and the Northern and
Northernmost regions are 99 km apart. There are 571 km separating the Southernmost
and Northernmost regions. I spaced each site within each region with 7 +/-1.2 km SE.
The two northern regions have established populations of Carcinus maenas but not of
Hemigrapsus sanguineus. Both species of crabs inhabit rocky intertidal areas within the
two southern regions; however, there is a greater abundance of H. sanguineus than C.
maenas in the Southernmost region than in the Southern region as of the sample dates. I
sampled snails haphazardly from the upper intertidal zone both on and under rocks at low
tide. Regions vary in coastal temperatures (Appendix A, Figure A.1); from May to
September temperatures are on average 4 degrees Celsius colder in the Northernmost
region than in the Northern region. These summer temperatures are only one-third of a
degree Celsius cooler in the Northern region versus the Southern region, and
temperatures are on average 5 degrees Celsius warmer in the Southernmost region in
20
comparison to the Southern region. The Southernmost region in the summer is on average
9.8°C warmer than the Northernmost region. These temperatures provide relative
estimates for regional differences, as intertidal temperatures will range more extremely
(much colder in the winter and warmer in the summer) than even near-shore buoys. All
sites are protected from intense wave exposure to support populations of crabs among
Ascophyllum nodosum and under small boulders at low tide.
Three sites were excluded from the analyses presented here due to extreme physical
differences from the other sites: in the Northern region the Inlet site in Winter Harbor
(few rocks and muddy substrate provided little structure for snail and crab shelter) and
Schoodic Point (steep cliff site), Winter Harbor (primarily cliff and steep ledges) were
eliminated. Avery Point from the Southernmost region was eliminated because the site
was comprised primarily of ledges rather than boulders. Wilbur Neck was kept in the
analysis although it was much more protected than the other sites because shell thickness
was statistically similar to a much more exposed site within the region, West Quoddy
Head (See Appendix B, Figure B.4).
Sample Sizes and Height Ranges for Examining Thickness of Unscarred Snails
No snails were collected that were less than 5 mm in shell height from the Southern
region nor greater than 14 mm in height from the Northernmost region. I focused my
analyses on unscarred snails that were between 5 and 13 mm in shell height, as the
number of snails found within each region was fairly even across regions (mean of
N=471+/- 59 SE) compared to several other possible size ranges (including 5 to 9 mm, 5
to 10 mm, and 5 to 14 mm), and L. saxatilis are most commonly less that 12 mm (Gosner
21
1978). I focused analyses on snails of similar heights, thus averages in shell height
calculated reflect the average heights of my cropped samples rather than aiming to
capture average heights in the field.
Snail Shell Thickness and Scarring Frequency
Samples were collected between 19 May 2005 and 6 June 2005 from 14 sites (Figure
1.1). I sorted all collected snails into three categories: unscarred (shell clear of scars,
chips, and blemishes), scarred (shell shows clear evidence of a crab predation attempt
either healed or unhealed (Figure 1.2)), and blemished (shell is marred or chipped in
some fashion but the origin of the damage is unclear). I scored shells with a distinct “U”
or “V” shaped scar as evidence of a previous crab attack.
Snails that were questionable or only faintly scarred were not considered to be crab
scarred and were categorized as blemished. As some shells scored as blemished may
have been attacked by crabs, the frequency of crab-scarred snails in each sample may
thus be underestimated. In examining snail morphology, I excluded blemished snails.
When calculating scarring frequency per site I counted blemished snails as a part of the
total unscarred portion of the sample.
As previously discussed, shell thickness is an important shell feature in determining a
snail’s vulnerability to predation by crabs. Shell thickness coupled with height was thus
used to compare shell morphology within and among regions. All shell thickness
measurements were made at the outermost point of the aperture using digital calipers; in
order to measure the aperture lip thickness consistently without breaking the curved edge
22
of the lip, I measured 1.0 mm inward from the edge of the outer lip of the aperture, by
marking the calipers appropriately.
Shell thickness measurements were log transformed for normality. As aperture
thickness changes as the snail grows, I looked at the relationship between shell thickness
and shell height. Since within-site and within-region differences were minimal, I focused
primarily on the differences among the four regions in this chapter (see Appendix B for
within region differences).
Crab Density and Biomass
Ten to twenty 0.5 m2 quadrats were placed haphazardly across a 25 m transect in the
Ascophyllum nodosum mid-to-upper intertidal zone during low tide to conduct crab
density measures (transects were placed parallel to the low tide edge). Crabs were found
among the macroalgae and under rocks; they were identified, sexed, measured, and
ovigerous females were noted. Quadrat data were collected between 19 May 2005 and 6
June 2005 from 12 of the 14 sites where snails were sampled. The 13th site was collected
on 20 July 2006 at Odiornes Point, NH (OP), and the Evergreen site in Winter Harbor,
ME (EV) was not sampled because the large boulders present there made it impossible to
sample for crabs. The biomass of each crab was calculated based on power curve
equations (H. sanguineus: y=0.000362x3.071420, R2 = 0.99; C. maenas: y =
0.000239x2.984721, R2 = 0.99; y=weight (g); x=carapace width (mm)) generated from
measuring and weighing crabs collected from Odiornes Point (20 January 2005) starved
for at least 72 hours.
23
Shell Thickness of Unscarred versus Scarred Snails
To understand how scarring in snail shells may influence future predation attempts, I
compared the shell thicknesses of unscarred and scarred snails at five of the most heavily
scarred sites (within the three most southern regions). To elucidate how a snail shell
changes as a result of healing from a scar, I measured the thickness of the shells at the
aperture and the distance from the aperture edge to the scars. To investigate whether scars
thicken the shell only at the scar or also thicken any subsequent shell growth, I compared
shell thickness in snails that were recently scarred (the scar is close to the aperture edge)
to snails that had been scarred earlier (the scar is further from the aperture edge) to
unscarred snails. Since I measured snails 1.0 mm into the aperture, I considered recent
scars to be 0.5-1.5 mm from the edge and old scars to be greater than 1.5 mm from the
aperture edge. I eliminated snails with scars less than 0.5 mm from the aperture edge
since they were too recently chipped and may not have had ample time to heal over to
form a scar.
Analyses
I examined relationships among mean shell thickness, mean shell height, mean crab
density, mean crab biomass, region, latitude, and temperature. Mean height and mean
shell thickness at the aperture were both normally distributed, and crab density and
biomass were log transformed (log(x+1)). I also compared the frequency of scarred snails
among regions to examine if this was related to mean shell thickness and mean crab
density. Mean scarring frequencies were normally distributed. Finally, I compared shell
thickness between scarred and unscarred snails and among scarred snails that had varying
24
history of crab encounters; some had been recently scarred (scar is close to the aperture
edge) and some had been scarred long ago (scar is far from the aperture edge). All
statistical analyses were performed with the statistical software JMP 5.1.
Results
Shell Thickness and Height Differences across Regions
The Northernmost region (Figure 1.3a) has significantly thinner unscarred snails
(0.15 +/- 0.003 mm SE, n=572) than the other three regions (ANCOVA, REstricted or
REsidual Maximum Likelihood (REML) Effect Tests: P=0.0010; LS Mean Differences
Tukey HSD: α=0.05). The three southern regions have similar shell thicknesses (Northern
region=0.29 +/- 0.006 mm SE, n=570 N; the Southern region=0.41 +/- 0.008 SE mm,
n=402; the Southernmost region=0.35 +/- 0.007 mm SE, n=339). I included height as a
covariate, region as a fixed effect, and site nested within region as a random effect (see
Table 1.1 for number of snails measured per size class per region.) Aperture thickness is
shown for snails of particular size classes of shell height (mm) for the four regions
(Figure 1.3b), and levels not connected by the same letter are significantly different from
one another (ANOVA: F5, 1882=235.8, P<0.00001, LS Means Differences Tukey HSD,
α=0.05). Shell thickness decreases with increasing latitude (Figure 1.4a, R2=0.50,
ANOVA: F1,13=11.8, P=0.0050), and increases with increasing temperature (Figure 1.4b,
R2 = 0.52, ANOVA: F1,13=12.9, P=0.0037).
The three southerly regions have similar shell heights (8.17 +/-0.262 mm SE, 9.77 +/0.439 mm SE, 8.52 +/- 0.474 mm SE from south to north respectively), and the
Southernmost, Northern, and Northernmost (7.45 +/-0.321 mm SE) regions also have
25
similar shell heights (Table 1.2, LS Means Differences Tukey HSD, α=0.05). While size
ranges were cropped to examine only shell features of snails 5 to 13 mm in shell height,
regions still reflect some differences in average heights. Additionally, as shell height
increases, aperture thickness also increases (Figure 1.5, R2=0.6745, ANOVA: F1,13=24.8,
P=0.0003), and the southerly region individual sites show a trend of having thicker and
taller shells than those from the northerly regions.
Crab Density and Biomass across Regions
Neither snail scarring frequency (Figure 1.6) nor snail shell thickness (Figure 1.7) is
significantly associated with crab density at a particular site (scarring frequency
ANOVA: F1,12=0.90, P=0.3643 and shell thickness ANOVA: F1,12=1.5, P=0.2439). There
is a greater density of crabs in the southernmost region than in the other three regions
with site as a nested random variable (2-5 sites per region) (Region effect: P=0.0090; LS
Means Differences Tukey HSD, α=0.05). These density figures include all intertidal
crabs which were comprised of only two species: H. sanguineus and C. maenas.
Hemigrapsus sanguineus was not found in either of the two northern regions, whereas I
found 95% H. sanguineus in the Southernmost region and 17% H. sanguineus in the
Southern region (Figure 1.8, Table 1.3). There is a greater biomass of crabs in the
Southernmost, Northern, and Northernmost regions than in the Southern, Northern, and
Northernmost regions with site as a nested random variable (2-5 sites per region) (Region
effect: P=0.0401; LS Means Differences Tukey HSD, α=0.05) (Figure 1.9, Table 1.4).
In the Southernmost region the average carapace width (CW) of H. sanguineus was
11.0 mm (+/- 0.26 mm SE, N=574) and the average CW for C. maenas was 30.2 mm (+/-
26
5.11 mm SE, N=10). In the Southern region the average carapace width (CW) of H.
sanguineus was 12.4 mm (+/- 1.89 mm SE, N=17) and the average CW for C. maenas
was 8.9 mm (+/- 0.33 mm SE, N=123). The average CW for C. maenas was 30.8 mm (+/1.02 mm SE, N=116) in the Northern region and 32.4 mm (+/- 0.86 mm SE, N=124) in
the Northernmost region. (See Tables 1.3 and 1.4 for details on crab density, biomass and
species within sites and regions.)
Shell Thickness and Scarring Frequency
Scarring frequency is highly dependant on the thickness of the shell (Figure 1.10, y=0.9396x2 + 0.7826x - 0.0663, R2 = 0.4793, ANOVA: F2,13=6.4, P=0.0147); scarring
frequency positively increases with shell thickness at the aperture in a curvilinear fashion
(R2 = 0.4793). There are no significant differences in mean scarring frequency across the
four regions with site as a nested random variable (2-5 sites per region; 14 sites total) (LS
Means Differences Tukey HSD, α=0.05), but there appears to be a trend that the
Northernmost region has a lower number of scars per site than the other three regions
when mean scarring frequency is plotted against latitude (Figure 1.11; y=-0.0177x2 +
1.5115x - 32.152, R2 = 0.42, ANOVA: F2,13=4.0, P=0.050). The percentage of blemished
snails ranged from 3.6 to 17.6 % of total snails collected per region (Table 1.5b). (See
Table 1.5 for details on scarring frequency within sites and regions.)
27
Shell Thickness of Unscarred versus Scarred Snails
To compare unscarred shell thickness to scarred shell thickness, I selected the five
sites which had scarring frequencies greater than 0.08 (Table 1.6). Scarred snails within
these sites are significantly greater in shell thickness at the aperture (0.42 +/-0.011 mm
SE, N=234) than unscarred snails (0.34 +/-0.006 mm SE, N=658) (Figure 1.12, Table
1.7; ANCOVA with height as a covariate, region as a fixed effect, and site nested within
region as a random effect: F 10,891=107.2, P=0.0284).
Since scarred snails heal as they grow, their shells are thick at the scar and then thin
out to the thickness of the rest of the shell (Figure 1.13, ANOVA: F2, 1053=8.1, P=0.0003,
LS Means Differences Tukey HSD, α=0.05). Shell thickness (log transformed) is greater
when the scar is close to the aperture than when it is farther away, but this relationship is
not significant. Snails that had been scarred further from the edge are statistically similar
in shell thickness to unscarred snails, although there appears to be a trend for thicker
shells in the old-scarred snails. Recently scarred snails are significantly thicker than
unscarred snails. Thickness of the shell decreases as the distance from the scar to the
aperture increases until the scar becomes greater than 1.5 mm from the aperture edge.
Discussion
Crab abundance and species diversity in New England has been ever-increasing
northward in recent years. Although in my 2005 survey, the Northern region had only
19% H. sanguineus versus C. maenas, this is a great increase since 2001, when H.
sanguineus was reportedly not established yet in this region (Tyrrell 1999, 2002). There
is a greater density of crabs, regardless of species, from south to north, and there is
28
overlap in crab density among the northern three regions. There is high overlap in total
crab biomass among all four regions although species composition varies from south to
north. Previous studies specify that northern New England had substantially lower crab
abundances than in more southerly regions (Seeley 1986, Trussell and Smith 2000,
Trussell and Etter 2001), however more recent studies have indicated that C. maenas has
substantially increased in abundance in Canada, north of my Northernmost region (Audet
et al. 2003). While current populations of C. maenas in Northeastern Maine remain
inconsistent (Seeley, R. H., personal communication), with an increase in temperature,
populations will likely become more consistently abundant. Additionally, sporadic pulses
in crab abundances may be associated with the evolution of thicker shells. For instance,
in 2001 there was a considerable increase in C. maenas abundance in Winter Harbor, ME
(Harris, L. G, personal communication), which may have influenced local snail
populations. Even if crab populations subsided in subsequent years, since L. saxatilis live
on average six years, populations may still have remained thicker.
It is not surprising that my 2005 crab density measures do not relate to snail scarring
frequency or shell thickness at a site, since my data are only a snapshot in time of crab
density at a site. More importantly, my surveys indicate the presence of intertidal crabs
and the relative proportions of these particular crab species within the range of L.
saxatilis. Long-term decadal trends would be important to consider in order to understand
more fully how crab abundances are specifically influencing shell thickness which will
then affect scarring frequency. Thinner shells in the Northernmost region suggest that
differences are likely primarily due to temperature as well as historical crab abundance
differences between the regions (see Figure 1.4), as Trussell and Etter (2001) reported for
29
congener L. obtusata. The Northernmost region has summer temperatures that are on
average four degrees Celsius colder than in the Northern region. Temperatures are similar
in the Northern and Southern regions corresponding with similar shell thicknesses, but
one would have expected shell thickness differences between the Southern and
Southernmost region, as these regions have an on average five degree Celsius
temperature difference in the summer. However, it is important to acknowledge that
temperature and crab abundance alone do not influence shell features; other abiotic and
biotic forces, including life history and genetic traits, together influence shell shape and
thickness.
Although L. saxatilis in the northern Gulf of Maine are thinner than populations
further south, perhaps northern snails are more efficient at depositing calcium carbonate
and thus grow at a faster rate. Trussell and Etter (2001) report that L. obtusata from the
northern Gulf of Maine are genetically disparate from southern Gulf of Maine
populations; they suggest that there may be a greater selection for rapid growth and
efficient shell accretion in the north, where water temperatures are much colder. Perhaps
the same is true for L. saxatilis, however, my investigations do indicate that even taller
snails from the north are significantly thinner than those matched for height from the
three southerly regions; even if it takes a shorter amount of time to reach this length, they
are still more vulnerable to predation due to their thin shells. In general, faster growth
generates a thinner shell, and thickly-shelled snails tend to grow at a slower rate (Harvell
1990).
Snails from the Northernmost region in the 11-13 mm size class are statistically
similar in shell thickness to snails from the Northern region in the 9-11 mm size class and
30
snails from all other regions in the 5-7 mm and 7-9 mm size classes (Figure 1.3b). Even
though tall snails from the Northernmost region are likely just as vulnerable as these
snails from other regions that are statistically similar in shell thickness, there are much
fewer snails in the Northernmost region that fall into this tall size class (see Table 1.1:
11-13 mm, N=11). If we consider one size class smaller in the Northernmost region (9-11
mm), these snails are statistically similar to snails from the Northern region in the 5-7
mm size class but thinner than all other snails from the smaller size classes from the other
three regions. Even if snails from the Northernmost region get to a greater height faster
than those from the south, they will still likely be more vulnerable than most snails from
further south.
As supported previously, scarring frequency is highly dependent upon shell thickness,
(Elner and Raffaelli 1980, Vermeij 1982a); crabs will have more difficulty in chipping a
thicker shell, so thicker shells are more likely to be left uneaten than thinner shells.
Similarly, my studies show that mean scarring frequency is positively related to mean
shell thickness at the aperture (Figure 1.10). The thinner the shell, the more likely a crab
will be successful in preying upon a snail, and thus the snail (and its shell) will be
destroyed. Similarly, with a thicker shell, a crab may have a more difficult time cracking
the snail, and thus may be more likely to leave a chipped and uneaten snail. Additionally,
as snails get older, they become thicker, and there is a greater likelihood for scars to be
present in an older snail that has had time to accumulate them over one or more years.
However, above 0.30 mm in shell thickness, scarring frequency is highly variable, and
these thick shells are largely found in the southern three regions. As shell thickness
increases even further, a crab may have such a difficult time in cracking the snail that it
31
will not be able to damage the shell at all, so scarring frequency may decrease slightly for
these very thick shells.
Although there are no significant differences in mean scarring frequency across the
four regions, there is a trend showing that snails from the northerly sites have fewer scars
and thinner shells than those from more southerly sites (Figure 1.11). Scarring frequency
tapers off in areas with very thick shells in the south because it may be difficult for some
crabs to leave any evidence of a predation attempt, and the negative polynomial
relationship is largely driven by the low rate of scarring frequency in the northernmost
region. This trend for lower scarring frequency in the Northernmost region may be
occurring for several possible reasons; (1) there are likely fewer crabs, (2) the crabs that
are there may have a lower metabolism, slowed by the lower temperature, and thus the
predation pressure is lower, and (3) predation encounters that do occur are likely always
lethal, due to the thinner shelled snails.
Recently scarred snails have significantly thicker shells than unscarred snails (Figure
1.13), so these scarred snails are likely to be less vulnerable to predation than unscarred
snails. Protection to the snail from the scar would likely only last for one or two growing
seasons, as the scar will grow further from the aperture over time and then thin out to the
thickness of an unscarred snail. Snails with sub-lethal injuries to their shell may be less
vulnerable to future predation attempts by crabs, due to their increased thickness at the
site of the scar. This is contrary to studies conducted where sublethal predation has been
associated with an increased risk of lethal predation (Meyer and Byers 2005 and
references therein). Greenfield et al. (2002) found that scarred Littoraria irrorata had
thicker shells than unscarred conspecifics, which provided scarred snails with a reduced
32
risk of predation by Callinectes sapidus, the blue crab. Future studies (Chapter III)
presenting scarred versus unscarred Littorina saxatilis to its crab predators will reveal
important information on whether scarred L. saxatilis are in fact less vulnerable to
predation.
Since crabs are increasing in abundance and species diversity in northern New
England, it is imperative to look at the vulnerability of their prey items. Here I show that
one of the most important features in determining a shelled prey item’s susceptibility to a
shell-breaking predator, shell thickness, varies among populations of L. saxatilis within
New England. Additionally, there is evidence that scarred snails may have a decreased
risk of future predation due to their likely thicker shell where the previous chip has
healed (Chapter III, Greenfield et al. 2002). In general thinner shelled snails in the north
will likely be more vulnerable to predation by crabs than thicker shelled conspecifics in
the south (Chapter II). As coastal temperatures warm, crabs are likely to increase in the
north, which may have a serious impact on local populations of L. saxatilis. Even if thinshelled northern snails are able to increase in shell thickness as a response to increasing
crab exposure and warmer waters, crabs may peak in abundance before snails have the
time or the physiological capacity to response phenotypically. It is of utmost importance
to document patterns of morphological variation among native prey that are likely
vulnerable to increasing and expanding populations of exotic predators.
33
Table 1.1 Number of snails measured per size class per region.
Southernmost
Southern
Northern
Northernmost
5-7
mm
7-9
mm
9-11
mm
11-13
mm
111
117
75
36
18
126
159
99
177
173
124
96
282
202
77
11
Table 1.2a Summary of each site: unscarred snails 5-13 mm in height: shell thickness
(mm) +/-SE, shell height (mm) +/-SE, and N (number of snails measured).
Site
Latitude
(°N)
SP
WP
RH
Southern
OP
KP
EV
WO
Northern
FR
MP
QU
CP
Northernmost EA
WN
PA
41.33
41.33
43.00
43.04
43.10
44.34
44.36
44.38
44.40
44.82
44.89
44.90
44.90
44.95
Region
Southernmost
Shell
Shell
Thickness SE
Height SE
N
(mm)
(mm)
0.32 0.011
7.91 0.171 152
0.38 0.010
8.44 0.132 187
0.47 0.018
10.64 0.149 105
0.46 0.008
9.28 0.125 159
0.32 0.011
9.38 0.132 138
0.31 0.017
9.55 0.255
89
0.32 0.012
9.10 0.164 173
0.24 0.009
7.52 0.170
91
0.27 0.008
7.92 0.135 217
0.23 0.012
8.54 0.177
94
0.13 0.006
7.16 0.147
91
0.11 0.004
7.70 0.132 132
0.16 0.003
6.62 0.064 168
0.09 0.005
7.22 0.140
87
Mean N=135+/-11.4SE
34
Table 1.2b Summary of each region: unscarred snails 5-13 mm in height: mean shell
thickness (mm) +/-SE, mean shell height (mm) +/-SE, and number of sites in each region
(N).
Region
Latitude
(°N)
41.33
43.05
44.37
44.89
Southernmost
Southern
Northern
Northernmost
Mean US
Shell
SE
Thickness
(mm)
0.35
0.028
0.42
0.049
0.28
0.025
0.15
0.023
Mean US
Shell
SE
N
Height
(mm)
8.17
0.262
2
9.77
0.439
3
8.52
0.474
4
7.45
0.321
5
Table 1.3a Summary of each site: mean crab density per 0.5 m2 at each site +/-SE, % C.
maenas (CM), % H. sanguineus (HS), and number of quadrats.
Region
Site
SP
WP
RH
Southern
OP
KP
WO
Northern
FR
MP
QU
CP
Northernmost EA
WN
PA
Southernmost
LatiMean Crab
tude
SE
Density
(°N)
41.33
10.70 2.450
41.33
47.70 9.185
43.00
0.82 0.536
43.04
4.96 0.562
43.10
1.50 0.886
44.36
1.63 0.483
44.38
2.87 1.320
44.40
0.88 0.328
44.82
0.13 0.091
44.89
0.40 0.190
44.90
2.40 0.729
44.90
0.20 0.092
44.95
3.90 0.827
%
CM
%
HS
9.3%
0%
77.8%
89.1%
83.3%
100%
100%
100%
100%
100%
100%
100%
100%
90.7%
100%
22.2%
10.9%
16.7%
0%
0%
0%
0%
0%
0%
0%
0%
# of
quadrats
10
10
11
12
8
30
15
25
15
15
15
20
20
Table 1.3b Summary of each region: mean crab density per 0.5 m2 at each region +/-SE,
% C. maenas (CM), % H. sanguineus (HS), and number of quadrats.
Lati# of
Mean Crab
Region
tude
SE
% CM % HS quadDensity
(°N)
rats
41.33
29.20 6.278
4.7% 95.3%
20
Southernmost
43.03
2.60 0.490 83.4% 16.6%
31
Southern
44.38
1.63 0.373
100%
0%
70
Northern
44.89
1.48 0.288
100%
0%
85
Northernmost
35
Table 1.4a Summary of each site: mean total crab biomass (g) per 0.5 m2 at each site +/SE, mean biomass (g) of C. maenas (CM), mean biomass (g) of H. sanguineus (HS), and
number of quadrats.
Region
Site
Latitude
(°N)
SP
WP
RH
Southern
OP
KP
WO
Northern
FR
MP
QU
CP
Northernmost EA
WN
PA
Southernmost
Mean Total
Crab
Biomass (g)
41.33
41.33
43.00
43.04
43.10
44.36
44.38
44.40
44.82
44.89
44.90
44.90
44.95
25.07
59.36
1.39
3.02
1.04
13.13
25.81
9.82
1.83
2.34
20.35
2.97
39.45
SE
8.674
13.09
1.145
0.803
0.439
4.79
14.79
5.195
1.483
1.395
7.345
2.973
9.965
Mean
Biomass
(g) of
CM
11.20
0
0.44
1.75
0.74
13.13
25.81
9.82
1.83
2.34
20.35
2.97
39.45
Mean
Bio# of
mass
quad(g) of
rats
HS
13.88
10
59.36
10
0.95
11
1.28
12
0
8
0
30
0
15
0
25
0
15
0
15
0
15
0
20
0
20
Table 1.4b Summary of each region: mean total crab biomass per 0.5 m2 at each region
+/-SE, mean biomass of C. maenas (CM), mean biomass of H. sanguineus (HS), and
number of quadrats.
Region
Southernmost
Southern
Northern
Northernmost
Latitude
(°N)
41.33
43.03
44.38
44.89
Mean Total
Crab
Biomass (g)
42.21
1.82
16.25
13.39
SE
17.143
0.612
4.876
7.388
Mean
Biomass
(g) of
CM
5.60
0.97
16.25
13.39
Mean
# of
Biomass
quad(g) of
rats
HS
36.62
20
0.84
31
0
70
0
85
36
Table 1.5a Summary of each site: number of snails collected, number of scarred snails
collected, and mean scarring frequency +/-SE, number of samples collected per site (N),
and percent of total snails collected blemished per site.
Region
Site
Southernmost
SP
WP
RH
OP
KP
EV
WO
FR
MP
QU
CP
EA
WN
PA
Southern
Northern
Northernmost
Total
Latitude
# of
(°N)
Snails
41.33
41.33
43.00
43.04
43.10
44.34
44.36
44.38
44.40
44.82
44.89
44.90
44.90
44.95
555
10936
647
2300
656
208
727
203
877
312
149
214
201
322
# of
Scarred
Snails
68
510
87
118
69
34
64
13
43
10
3
5
0
5
Mean
%
Scarring
N SE
BlemFrequished
ency
0.126
3 0.015
17.8
0.050 20 0.004
11.9
0.134
1
0
24.6
0.045
5 0.010
14.9
0.105
1
0
13.4
0.163
1
0
26.0
0.089
2 0.006
16.0
0.064
1
0
12.8
0.048
2 0.008
7.8
0.032
1
0
5.1
0.020
1
0
1.3
0.021
2 0.004
0.5
0.000
1
0
8.0
0.016
1
0
3.1
Table 1.5b Summary of each region: number of unscarred snails collected, number of
scarred snails collected, mean scarring frequency +/-SE, number of sites in each region
(N), and average percent of total snails collected blemished per region.
Total Total Mean
%
Latitude
# of
# of
Scarring
SE
N Blem(°N)
US
S
Frequency
ished
41.33 11491
578
0.088 0.0277 2
14.9
Southernmost
43.05 3603
274
0.095 0.0226 3
17.6
Southern
154
0.091 0.0196 4
15.6
44.37 2015
Northern
44.89 1198
23
0.018 0.0175 5
3.6
Northernmost
Region
37
Table 1.6 Mean scarring frequencies at the five most heavily scarred sites within the
three southern regions.
Region
Site
Southernmost
Southern
Southern
Northern
Northern
SP
RH
KP
EV
WO
Latitude Mean Scarring
(°N)
Frequency
41.33
0.126
43.00
0.134
0.105
43.10
0.163
44.34
44.36
0.089
Table 1.7 Number of snails measured per size class per region for scarred and unscarred
snails. Snail measurements were treated as continuous variables for statistical analyses
and separated into size classes only for graphical purposes.
5-7
mm
Scarred
Unscarred
7-9
mm
8
133
9-11
mm
32
166
11-13
mm
98
96
194
165
38
○
C. maenas only
●
C. maenas and
H. sanguineus
o Wilbur Neck (WN)
o West Quoddy Head
(QU)
o Passamaquoddy (PA)
o Eastport Harbor (EA)
o Comstock Point (CP)
Northernmost
Northern
o
o
o
o
N
Mermaid’s Purse (MP)
Wonsqueak (WO)
Frazer (FR)
Evergreen (EV)
Southern
• Kittery Point, ME (KP)
• Rye Harbor, NH (RH)
• Odiornes Point, NH
(OP)
Southernmost
0
100
200 km
• Weekapaug Point, RI
(WP)
• Stonington Point, CT
(SP)
Figure 1.1 Map of four regions where sampling occurred: Northern Maine
(Northernmost) 44.89 ºN, Mid-coast Maine (Northern) 44.38 ºN, New Hampshire and
Southern Maine (Southern) 43.05 ºN, and Rhode Island/Connecticut border
(Southernmost) 41.33 ºN. Sites with C. maenas only (○); sites with both C. maenas and
H. sanguineus (●). Individual sites (and site codes) are listed from North to South in the
boxes to the right of the map.
39
A
C
B
D
E
F
Figure 1.2 Scars as evidence of previous crab predation attempts on L. saxatilis shells:
(A) smooth “U”-shaped chips healed, (B) “V”-shaped chip healed, and most common
(C), (D), (E), and (F) one smooth “U”-shaped chip healed.
40
Aperture Thickness (mm)
Southernmost
1
Southern
Northern
0.8
Northernmost
0.6
0.4
0.2
0
5
7
9
11
13
Height (mm)
Figure 1.3a Aperture thickness (mm +/-SE) was plotted against shell height (mm) for
the four regions. Model incorporates region as a fixed effect, height as a covariate, and
site as a random effect nested within region. The Southernmost region is shown in black
diamonds, Southern region is shown in dark grey squares, the Northern region is shown
in light gray triangles, and the Northernmost region is shown in open circles. The
Northernmost region (dotted line) has significantly thinner shells than the other three
regions (ANCOVA, REstricted or REsidual Maximum Likelihood (REML) Effect Tests:
P=0.0010; LS Mean Differences Tukey HSD: α=0.05; solid lines: Northern, Southern,
Southernmost in order from thinnest to thickest). Thickness data were log transformed to
normalize distribution but presented in the figure with non-transformed measurements.
41
Aperture Thickness (mm)
0.7
Southernmost
0.6
Southern
AB
A
Northern
0.5
Northernmost
0.4
E E
EF
0.3
F
F
EFG
G
G
0.2
BC
BC
CD
DE
I
H
5-7 mm
7-9 mm
0.1
0
9-11 mm
11-13 mm
Mean Aperture Thickness
(mm)
Figure 1.3b Aperture thickness (mm +/-SE) is shown for snails of particular size classes
of shell height (mm) for the four regions. The Southernmost region is shown in black
bars, Southern region is shown in dark grey bars, the Northern region is shown in light
gray bars, and the Northernmost region is shown in open bars (see Table 1.1 for number
of snails measured per size class per region). Aperture thickness was log transformed to
normalize its distribution but shown in the figure as non-transformed data (ANOVA: F5,
1882=235.8, P<0.00001). Levels not connected by same letter are significantly different
(LS Means Differences Tukey HSD, α=0.05).
0.60
0.50
0.40
0.30
0.20
0.10
0.00
41
42
43
44
45
o
Latitude ( N)
Figure 1.4a Mean snail shell thickness at the aperture (mm +/-SE) decreases across
latitude (y=-0.0663x + 3.18, R2=0.50, ANOVA: F1,13=11.8, P=0.0050). Each point
represents an average shell thickness per site.
42
Mean Aperture Thickness
(mm)
0.6
0.5
0.4
0.3
0.2
0.1
0
8
12
16
20
o
Temperature ( C)
Mean Aperture Thickness
(mm)
Figure 1.4b Mean snail shell thickness at the aperture (mm +/-SE) increases across
temperature (°C) (y=0.0258x - 0.0584, R2 = 0.5162, ANOVA: F1,13=12.9, P=0.0037).
Each point represents an average shell thickness per site. NOAA temperature data were
used on a regional basis.
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Southernmost
Southern
Northern
Northernmost
6
7
8
9
10
11
Mean Height (mm)
Figure 1.5 Mean shell thickness at the aperture (mm +/-SE) is positively related with
mean height (mm +/-SE) (y=0.0879x - 0.4619, R2=0.6745, ANOVA: F1,13=24.8,
P=0.0003). Each point represents an average per site, and points are grouped by regions:
Southernmost region (open triangles); Southern region (open squares); Northern region
(filled triangles); Northernmost region (filled squares). The three southerly regions have
similar shell heights, and the Southernmost, Northern, and Northernmost regions also
have similar shell heights (LS Means Differences Tukey HSD, α=0.05).
43
Mean Scarring Frequency
Southernmost
Southern
Northern
0.10
Northernmost
-3
-1
1
3
-0.05
2
Log Mean Crab Density per 0.5 m
Mean Aperture Thickness
(mm)
Figure 1.6 Mean scarring frequency (+/-SE) is not significantly related to log
transformed mean crab density (+/-SE) per 0.5 m2 (ANOVA: F1,12=0.90, P=0.3643).
Each point represents an average per site, and points are grouped by region:
Southernmost region (open triangles); Southern region (open squares); Northern region
(filled triangles); Northernmost region (filled squares).
Southernmost
Southern
0.4
Northern
Northernmost
0.2
0.0
-3
-1
1
3
2
Log Mean Crab Density per 0.5 m
Figure 1.7 Mean shell thickness at the aperture (mm +/-SE) is not significantly related to
log mean crab density (+/-SE) per 0.5 m2 (ANOVA: F1,12=1.5, P=0.2439). Each point
represents an average per site, and points are grouped by region: Southernmost region
(open triangles); Southern region (open squares); Northern region (filled triangles);
Northernmost region (filled squares).
44
2
Log (Mean Crab Density +1) per 0.5 m
1.8
H. sanguineus
1.5
C. maenas
1.2
0.9
0.6
0.3
0
SP WP RH OP KP FR WO MP CP EA PA QU WN
SouthernSS
most
Southern
S
Northern
N
Northernmost
NN
Log (Mean Crab Biomass +1) per 0.5 m
2
Figure 1.8 Log transformed mean crab density per 0.5 m2 across sites within the four
regions. The proportion of each crab species is shown for each site; open bars for C.
maenas and grey bars for H. sanguineus.
2.5
H. sanguineus
C. maenas
2
1.5
1
0.5
0
SP WP RH OP KP FR WO MP CP EA PA QU WN
SouthernSS
most
Southern
S
Northern
N
Northernmost
NN
Figure 1.9 Log transformed mean crab biomass per 0.5 m2 across sites within the four
regions. The proportion of each crab species is shown for each site; open bars for C.
maenas and grey bars for H. sanguineus.
45
Mean Scarring Frequency
0.16
Southernmost
Southern
0.12
Northern
Northernmost
0.08
0.04
0
0
0.1
0.2
0.3
0.4
0.5
Mean Aperture Thickness (mm)
Mean Scarring Frequency
Figure 1.10 Mean scarring frequency (+/-SE) is positively related to mean shell
thickness at the aperture (mm +/-SE) (y=-0.9396x2 + 0.7826x - 0.0663, R2 = 0.4793,
ANOVA: F2,13=6.4, P=0.0147). Each point represents an average per site, and points are
grouped by region: Southernmost region (open triangles); Southern region (open
squares); Northern region (filled triangles); Northernmost region (filled squares).
0.16
0.12
0.08
0.04
0
41
42
43
44
45
o
Latitude ( N)
Figure 1.11 Mean scarring frequency against latitude has a negative curvilinear trend
from south to north (y=-0.0177x2 + 1.5115x - 32.152, R2 = 0.42, ANOVA: F2,13=4.0,
P=0.050).
46
Log Aperture Thickness (mm)
5-7 mm
7-9 mm
9-11 mm
11-13 mm
0
-0.15
-0.3
-0.45
-0.6
-0.75
Scarred
Unscarred
Figure 1.12 Unscarred snails had thinner shells at the aperture (mm +/-SE, log
transformed) than scarred snails (ANOVA: F 3,891=282.3, P=0.0290). Data from the five
most heavily scarred sites were grouped into size classes for graphical purposes, however
data were left as continuous variables for statistical analyses.
47
Aperture Thickness (mm)
A
0.5
AB
0.4
38
B
148
868
Recent Scar
(close)
Old Scar
(far)
Unscarred
(0.5-1.5 mm)
(1.5-3.5 mm)
0.3
0.2
0.1
0
Figure 1.13 Shell thickness of snails with scars close (0.5-1.5 mm) to the aperture edge
(filled bar), with scars far (1.5-3.5 mm) from the aperture edge (grey bar), and without
scars are compared (open bar). Shell thickness (mm +/-SE, log transformed) is greater
when scar is close to the aperture than when a shell is unscarred. (ANOVA: F2, 1053=8.1,
P=0.0003). Levels not connected by same letter are significantly different (LS Means
Differences Tukey HSD, α=0.05), and number of snails measured is shown at the top of
each bar.
48
CHAPTER II
VARIATION IN LITTORINA SAXATILIS MORPHOLOGY ACROSS
THE GULF OF MAINE: SNAIL VULNERABILITY TO CRAB PREDATION
Abstract
The introduced predatory Asian shore crab Hemigrapsus sanguineus has the potential
to impact the native snail Littorina saxatilis along the Atlantic coast of North America,
overlapping as it does in both the habitat and range of the snail. Snail populations located
500 km apart in New Hampshire and Maine are significantly different in shell shape, and
northern individuals have significantly thinner shells and are more vulnerable to crab
predation. As H. sanguineus continues to spread and increase in abundance,
understanding the vulnerability of native prey will be critical to elucidating potential
impacts on intertidal community structure.
Introduction
Prey employ chemical, morphological and behavioral defenses to contend with
predation. The expenditure of energy on these mechanisms must be balanced with energy
required for metabolism, growth, and reproduction. Defenses may already be in place
because they are always present (constitutive) or defenses may be plastic and produced
when exposed to a predator (inducible) (Harvell 1986). When predators are introduced to
an area, their prey may not have developed appropriate defense mechanisms, or the prey
49
may have had prior experience with a similar predator which may have required defense
responses (Freeman and Byers 2006).
Shelled gastropods provide clear evidence for evolution influencing morphological
changes over time and space (Vermeij 1987). Snails in the presence of shell-breaking
predators often will develop thicker shells through adaptation or phenotypic plasticity
(Ebling et al. 1964, Hughes and Elner 1979, Johannesson 1986, Appleton and Palmer
1988, Trussell 2000), since thicker shells are often stronger shells (Vermeij 1987).
Variation occurs through the influence of not only predation but also other biotic
influences, such as competition. Abiotic factors also play an important role in
development and adaptation, and may interact with biotic factors: for example, calciumcarbonate shells are formed more efficiently (requiring less energy) in warmer
environments (Vermeij and Currey 1980a), but the need to produce thicker shells in the
face of predation may cost more energy in colder environments. Wave-energy may also
influence morphology: snails in wave-exposed, high energy environments spend more
energy on the development of a large foot to grip the substrate than snails in low-energy
systems (Etter 1988, Trussell 1997).
The common intertidal periwinkle Littorina saxatilis displays a latitudinal difference
in shell thickness in the Gulf of Maine (Chapter I). Shells in the northern portion of the
Gulf of Maine are much thinner than those in the southern portion. Crab predators occur
throughout this range, and include a mixture of native species, an older crab invader, and
a newer crab invader. Potential native crab predators include xanthid and cancrid crabs
(Gosner 1978), however the two newest exotic species now dominate the intertidal rocky
shores of most of New England. The portunid, European shore crab, Carcinus maenas,
50
has been present along the Gulf of Maine coast since the 1890’s. The grapsid, Asian
shore crab, Hemigrapsus sanguineus, which was first discovered on the New Jersey shore
in 1988, moved north to southern Cape Cod by 1992, and crossed north and east into the
Gulf of Maine by 1998 (McDermott 1998, Tyrrell and Harris 1999), arriving in 2005 on
the central Maine coast in Acadia. South of Cape Cod, Hemigrapsus has apparently
replaced Carcinus in many intertidal locations (McDermott 1998, Ahl and Moss 1999,
Lohrer and Whitlatch 2002a). In the southern Gulf of Maine, both crab species now cooccur in rocky intertidal areas, but Carcinus remains as of 2006 the most abundant
intertidal crab.
Objectives
I tested whether Littorina saxatilis in the northern Gulf of Maine differ in
vulnerability to predation by H. sanguineus from snails in the southern Gulf of Maine by
examining their variability in shell structure and by measuring crab preference when
given a choice between snails from the two locations. I hypothesize that northern Gulf of
Maine snails will be both thinner-shelled and more vulnerable to predation than
conspecifics from the South.
Methods
Snail Morphology
As there appears to be variation in morphology across latitude for Littorina saxatilis, I
chose two sites (Figure 2.1) separated by 1.86 degrees in latitude to examine geographic
differences in snail morphology and vulnerability to crab predation. The northeastern-
51
most site in Maine is Wilbur Neck (WN), located within Cobscook Bay (44.90°N 67.15°W) and is protected from intense wave exposure. The southern site is in New
Hampshire at Odiornes Point (OP) (43.04°N -70.71°W) and is semi-exposed. The sites
differ in temperature regime; coastal water in WN is about 5 °C colder in the summer
(May to September) temperatures than in OP (Appendix A, Figure A.1). However both
sites have similar salinities largely ranging from 30 to 33 ppt
(http://www.ndbc.noaa.gov/).
From March to April 2005, I collected snails from the upper-intertidal zone of each
site and measured shell height, spire height, shell width, shell thickness at the aperture
(Figure 2.2), dry shell weight, and dry tissue weight. Only snails with intact shells (shells
without blemishes, chips, and scars) were used for these measurements. Shell dimensions
were measured using digital calipers. In order to measure the aperture lip thickness
consistently without breaking the curved edge of the lip, I measured 1.0 mm inward from
the edge of the outer lip of the aperture, by marking the calipers appropriately.
All measurements were checked for normal distributions. Dry tissue weight and dry
shell weight were normally distributed, and for these two measurements, I restricted the
pool of studied shells to shell heights between 5.5 and 7.5 mm because most of the
dissected snails (haphazardly selected from the sample) fell between these shell heights.
For shell thickness and width site-to-site comparisons, snails used were 5.0 to 8.0 mm in
height because most of the snails collected were within these heights. The mean shell
height was 6.8 mm (+/-0.101 mm SE; n=62) for OP snails and 6.5 mm (+/-0.059 mm SE;
n=126) for WN snails, but all analyses used height as a covariate. Shell thickness, width,
height and spire height measurements were all normally distributed. For all snail
52
morphological measurements, I compared the two sites (WN and OP) using analysis of
covariance (ANCOVA), with site as a fixed effect and height as a covariate. If the
interaction between site and height was significant, I performed a least squares (LS)
means (adjusted means) contrast between the sites. All statistical analyses were
performed with the statistical software JMP 5.1.
Snail Vulnerability to Predation
To test whether snails of differing thickness are differentially vulnerable to predation,
I performed a series of predation trials using male H. sanguineus. Male crabs were used
to keep claw sizes more consistent, as especially for this species, there is a sexual
dimorphism in claw size (female crabs have much smaller claws than males). I limited
the present studies to one species, the species that currently occurs at the southerly site
(OP) but not yet at the northerly site (WN). Crabs were starved for 48 hours prior to the
trials. Then crabs 22.6 to 31.0 mm in carapace width (mean 26.8 +/- 0.34 mm SE; all
crabs were statistically similar in carapace width) were placed in microcosm feeding
chambers (500 ml of aerated 35 ppt seawater in plastic 750 ml Ziploc® containers). Five
to ten chambers at a time were placed in a large covered opaque chamber (39-gallon
plastic Sterilite® storage bin) which did not permit the entry of light or other visual
stimuli. One OP and one WN snail (matched for height +/- 0.04 mm SE) were placed in
each chamber. Snails ranged from 5.8 to 7.8 mm in height (mean 6.5 +/-0.06 mm SE) and
were sized proportionally to the carapace width of the crab used per trial in order to
minimize variability in crab to snail size ratio. I wanted to examine crabs’ behavior when
53
presented with a large snail close to the maximum size the crab could successfully prey
upon.
Once snails were placed in each feeding chamber, I removed the cover to the large
chamber to examine the snails every 5 minutes for damage or mortality. Each trial was
terminated when the first snail was eaten. If both snails were eaten within any five minute
period, the trial was eliminated from the analysis. If neither snail was eaten after a one
hour period, the trial was eliminated. A total of 41 crab trials (using unique crabs that
chose one snail first over another snail) were used in the analysis (Chi Square Goodness
of Fit).
Results
Snail Morphology
Southern snails from Odiornes Point, New Hampshire (OP) have significantly greater
dry tissue weight than snails from Wilbur Neck, Maine (WN) (LS means contrast:
P<0.0001; Figure 2.3, Table 2.1). As snails increase in height their tissue weight also
increases, and this relationship is significantly different between the sites (P=0.0108,
Table 2.1). Snails from OP have significantly greater dry shell weight than snails from
WN (LS means contrast: P<0.0001; Figure 2.4, Table 2.1). As snails increase in height
their shell weight also increases, and this relationship is significantly different between
the sites (P=0.0004, Table 2.1)
Snails from OP are significantly wider than snails from WN (LS means contrast:
P<0.0001; Figure 2.5, Table 2.2). As snails increase in height their shell width also
increases, and this relationship is significantly different between the sites (P=0.0062,
54
Table 2.2). Snails from OP have significantly thicker shells than those from WN while
controlling for height (ANCOVA: P<0.0001; Figure 2.6, Table 2.2). The mean aperture
shell thickness is 0.36 mm (+/-0.010 mm SE; n=62) for OP snails and 0.17 mm (+/-0.003
mm SE; n=126) for WN snails. As snails increase in height their shell thickness at the
aperture also increases, and this increasing relationship is not statistically different
between the sites for snails 5-8 mm in shell height; there is no interaction between height
and site (ANCOVA: P=0.2913, Table 2.2a). Snails from WN have significantly taller
spires than snails from OP (LS means contrast: P<0.0001; Figure 2.7, Table 2.2). As
snails increase in height their spire height also increases, and this relationship is
significantly different between the sites (P=0.0049, Table 2.2). (See Figure 2.8 for a
visual comparison of snails from the two sites matched for height.)
Snail Vulnerability to Predation
Out of 41 trials during which Hemigrapsus sanguineus were observed eating one
snail before another snail, the crabs chose the northeastern Maine (WN) snail first over
the New Hampshire (OP) snail during 33 (80.5%) of the trials. Thus, crabs chose the
thinner-shelled WN snail over the thicker-shelled OP snail in a significantly greater
number of trials (χ2=8.4, P<0.005, DF=1) (Zar 1999). (See Appendix C for results of 36
additional trials performed.)
Discussion
Phenotypic characteristics may be highly variable among and within populations, as
they are potentially influenced by many local biotic and abiotic forces. Species that have
55
low larval dispersal often exhibit high variability in morphology as the result of local
shaping forces. The rough periwinkle, Littorina saxatilis, broods its young and has been
shown to exhibit high variability across its range and even within a site based on its
habitat (Janson and Sundberg 1983, Johannesson 2003, Grahame et al. 2006). I studied
the morphology of this prey species, L. saxatilis, at two sites separated by about 400 km.
Since L. saxatilis are ovoviviparous, little gene flow is expected to occur between
populations many kilometers apart. Populations of this snail in northeastern Maine and
New Hampshire are significantly different in shell morphology (Figure 2.8). Although
the morphological variables that describe shell shape cannot be divorced from one
another (Vermeij 1987), the individual variables: dry tissue weight, dry shell weight,
width, shell thickness at the aperture, and spire height are all significantly different
between the sites. Since snails from WN are narrower, so there is less room for tissue
growth, thus snails from WN have less tissue weight than OP snails, which are wider.
Snail behavior reflects this difference in shell morphology as well: snails from WN,
which have lighter, narrower, and thinner shells, move faster than those from OP with
heavier, wider, and thicker shells (Appendix D). Fast crawling behavior may be
beneficial to snails vulnerable to Nucella lapillus, the predatory dogwhelk (Harris, L.G.
personal communication, Matthews-Cascon 1997); however, crawling fast would not
likely benefit a snail matched with a crab predator.
In addition to these populations being separated by a great distance, which limits or
largely prevents gene flow, local abiotic and biotic factors at each site likely explain their
morphological differences. The sites chosen in this study differ in temperature regime,
wave exposure, and crab species assemblage. Although the locations have comparable
56
crab abundances in the region as a whole (Chapter I, Figure 1.8), the individual sites
differ in crab abundance (Chapter I, Table 1.3). Seeley (1986) examined L. obtusata in
the Gulf of Maine and found that previous to the invasion of C. maenas snails had thinner
shells with a higher spire, suggesting that the crabs’ presence stimulated rapid selection
for thicker shells with a lower spire. Littorina saxatilis shells in the north have both a
higher spire and thinner shells (Figures 2.6, 2.7 and 2.8), which makes them more
vulnerable to crab predation, as shown in my predation trials. Perhaps differences in crab
abundance accounts for differences in shell morphology, as selection pressures may be
reduced in the north due to lower crab abundance.
The colder temperatures (by as much as 5°C during prime growth season) in
northeastern Maine (Appendix A, Figure A.1) may also explain the thinner snail shells in
comparison to New Hampshire snails, as less calcium carbonate is accreted in colder
temperatures (Vermeij and Currey 1980b). As noted earlier, snails exposed to greater
wave action allocate more energy to body size in order to more firmly grip to the
substrate (Etter 1988), and less energy to shell thickness (Ebling et al. 1964, Hughes and
Elner 1979, Johannesson 1986) because there are typically fewer predators at more
exposed sites. However, the study sites I chose have wave exposure/predator regimes
contrary to expected scenarios. Odiornes Point is a semi-exposed site with predators
present, and Wilbur Neck is a protected site with a low number of predators present, and I
expected a more equal number of predators at the two sites (see Chapter I, Figure 1.8).
The reduced number of predators in Wilbur Neck is likely due to the fact that this site is
more liable to freeze in the shallow waters than a more exposed site in the region, such as
Eastport (EA) (Chapter I, Figure 1.1) (Harris, L.G., personal communication). During
57
warming trends in water temperature, C. maenas is likely to increase in number in these
shallow bay sites, which occurred in 1995 when Matthews-Cascon (1997) sampled in this
particular area of the bay. In order to more precisely understand differences in shell
thickness between these two areas, examining snails in sites of high wave exposure (with
potentially lower crab abundance) and sites of low wave exposure (with potentially
higher crab abundance) in both regions will be of value. Even at Eastport (EA), a site
with similar wave exposure and predator abundance to that of Odiornes Point (Appendix
A, Table A.4), shells are even thinner than those from WN (Appendix A, Table A.3; EA:
0.11 +/-0.004 mm SE; WN: 0.16 +/- 0.003 mm SE). Snails in the north in general are
thinner likely due to both temperature differences and historical crab abundance
differences (see General Introduction and Chapter I).
Shell thickness is one of the most important features in determining vulnerability to
predation (Vermeij 1987); thinner shelled individuals from WN are more vulnerable to
predation by crabs than those from OP. Of interest is that the newly introduced crab H.
sanguineus has larger claws than a C. maenas of equal carapace width (Appendix E),
which indicates that H. sanguineus may have an equal or greater effect on prey items than
C. maenas. A fairer comparison of H. sanguineus and C. maenas compares the per capita
effect of crabs that are likely to influence L. saxatilis of the common size range.
Comparisons of these sized crabs (~22-30 mm for H. sanguineus and ~44-56 mm for C.
maenas) show that both species have similar sized claws (Figure E.2). In practice, H.
sanguineus is suspected to have a larger effect because in the intertidal zone where L.
saxatilis is common, the expected crabs to overlap here would be small C. maenas that
would not likely have the ability to crack L. saxatilis and H. sanguineus that would
58
become large enough here to successfully prey upon the snails (larger C. maenas would
move to the lower intertidal and subtidal zone and overlap with L. saxatilis for a shorter
period of time during high tide). However more detailed analyses of internal musculature
would be necessary to compare crabs of different genera more precisely (Vermeij 1977,
Taylor 2000, 2001). In areas where H. sanguineus has replaced C. maenas in the
intertidal zone, impacts on invertebrate populations are comparable or even greater than
when C. maenas was the only crab invader on the shores, likely due to the greater
densities of the newer invader (Lohrer and Whitlatch 2002b). As H. sanguineus move
further northward with warming ocean temperatures, thinner shelled L. saxatilis in the
north may be more vulnerable to crab predation than conspecifics from the south. In turn,
however, there is the potential that, as the oceans warm, northern snails may be able to
increase calcium carbonate deposition, and thus have a modicum of hope as a new
predator arrives.
59
Table 2.1a ANCOVAs for snail dry tissue and shell weight with shell height as the
covariate between snails from Odiornes Point and Wilbur Neck.
Dry Tissue
Dry Shell
Weight
Weight
F
P
F
P
39.3 <0.0001 293.1 <0.0001
Site
21.3 <0.0001 67.9 <0.0001
Height
Site*Height 7.3 0.0108 15.7 0.0004
DF: Whole
Model,
3, 37
3, 37
Corrected
Total
LS means
F1,34=39.3,
F1,34=293.1,
contrast for
P<0.0001
P<0.0001
Site
Table 2.1b Differences in L. saxatilis dry tissue and shell weight between Odiornes
Point and Wilbur Neck (N=the number of observations).
Odiornes
Point, NH
Wilbur
Neck, ME
N
Dry Tissue
Weight (g)
mean
+/-SE
Dry Shell Weight
(g)
mean
+/-SE
14
0.0038
0.00049
0.0701
0.00470
24
0.0015
0.00011
0.0258
0.00125
60
Table 2.2a ANCOVAs for shell width, thickness and spire height with shell height as
the covariate between snails from Odiornes Point and Wilbur Neck.
Shell
Thickness at
the Aperture
F
P
F
P
62.5 <0.0001 560.7 <0.0001
Site
170
Height
<0.0001 18.8 <0.0001
6.5
1.1
0.2913
Site*Height 7.7 0.0062
DF: Whole
Model,
3, 187
3, 187
Corrected
Total
LS means
F1,184=62.5,
N/A
contrast for
P<0.0001
Site
Shell Width
Spire Height
F
666.1
P
<0.0001
308.9
<0.0001
8.1
0.0049
3, 187
F1,184=666.1,
P<0.0001
Table 2.2b Differences in L. saxatilis shell width, thickness and spire height between
Odiornes Point and Wilbur Neck (N=the number of observations).
N
Odiornes
Point, NH
Wilbur
Neck, ME
Shell Width
(mm)
Shell Thickness
at the Aperture
(mm)
mean
+/-SE
mean
+/-SE
62
6.12
0.094
0.36
126
5.64
0.049
0.17
Spire Height
(mm)
mean
+/-SE
0.010
1.60
0.041
0.003
2.26
0.026
61
WN
N
OP
0
100
200 km
Figure 2.1 Field sites: Wilbur Neck in northeastern Maine (WN) and Odiornes Point,
New Hampshire (OP).
62
A
B
C
D
Figure 2.2 Snail dimension measurements diagram: (A) spire height, (B) width, (C)
shell thickness, and (D) height (drawing adapted from Janson and Sundberg 1983).
63
Dry Tissue Weight (g)
0.008
OP
0.007
WN
0.006
0.005
0.004
0.003
0.002
0.001
0
5.5
6
6.5
Height (mm)
7
7.5
Figure 2.3 Dry tissue weight (g) against height (mm) for L. saxatilis specimens from
Odiornes Point, NH (OP) (filled diamonds, solid line) y=0.0023x – 0.0111, R2=0.413,
P=0.0132 and from Wilbur Neck, Northeastern Maine (WN) (open diamonds, dashed
line) y=0.0006x – 0.0023, R2=0.362, P=0.0019. Snails from OP have significantly greater
dry tissue weight than snails from WN (LS means contrast: P>0.0001, Table 2.1).
Dry Shell Weight (g)
0.12
OP
0.1
WN
0.08
0.06
0.04
0.02
0
5.5
6
6.5
Height (mm)
7
7.5
Figure 2.4 Dry shell weight (g) against height (mm) for L. saxatilis specimens from
Odiornes Point, NH (OP) (filled diamonds, solid line) y=0.0279x -0.1131, R2=0.669,
P=0.0004 and from Wilbur Neck, Northeastern Maine (WN) (open diamonds, dashed
line) y=0.0098x – 0.0365, R2=0.703, P<0.0001. Snails from OP have significantly greater
dry shell weight than snails from WN (LS means contrast: P>0.0001, Table 2.1).
64
7.5
OP
Width (mm)
7
WN
6.5
6
5.5
5
4.5
4
5
6
7
8
Height (mm)
Aperture Thickness (mm)
Figure 2.5 Shell width (mm) against height (mm) for L. saxatilis specimens from
Odiornes Point, NH (OP) (filled diamonds, solid line) y=0.8967x + 0.0482, R2=0.918,
P<0.0001 and from Wilbur Neck, ME (WN) (open diamonds, dashed line) y=0.784x +
0.5493, R2=0.891, P<0.0001. Snails from OP are significantly wider than snails from WN
(LS means contrast: P>0.0001, Table 2.2).
0.6
OP
0.5
WN
0.4
0.3
0.2
0.1
0
5
6
7
8
Height (mm)
Figure 2.6 Shell thickness at the aperture (mm) against height (mm) for L. saxatilis
specimens from Odiornes Point, NH (OP) (filled diamonds, solid line) y=0.0271x +
0.1734, R2=0.0808, P=0.0252 and from Wilbur Neck, Northeastern Maine (WN) (open
diamonds, dashed line) y=0.0165x + 0.0649, R2=0.132, P<0.0001. Snails from OP have
significantly greater shell thickness at the aperture than snails from WN (ANCOVA:
P>0.0001, Table 2.2).
65
Spire Height (mm)
3
OP
WN
2.5
2
1.5
1
0.5
5
6
7
Total Shell Height (mm)
8
Figure 2.7 Spire height (mm) against total height (mm) for L. saxatilis specimens from
Odiornes Point, NH (OP) (filled diamonds, solid line) y=0.2811x - 0.2998, R2=0.4849,
P<0.0001 and from Wilbur Neck, Northeastern Maine (WN) (open diamonds, dashed
line) y=0.3899x - 0.2764, R2=0.7462, P<0.0001. Snails from WN have significantly taller
spires than snails from OP (LS means contrast: P>0.0001, Table 2.2).
A
B
Figure 2.8 Snails matched for length (6.85 mm ±0.05) from (A) Odiornes Point, New
Hampshire (OP) and (B) Wilbur Neck, Maine (WN).
66
CHAPTER III
DOES SUB-LETHAL INJURY PROVIDE PREY WITH A REDUCED RISK OF
LETHAL PREDATION?
Abstract
Although scars are the physiological repairs of chipped shells, they also may confer
subsequent advantage to snails against future attack. Crabs spend a longer time handling
scarred Littorina saxatilis over unscarred conspecifics; however, this trend is only
significant when considering the most recent intertidal crab invader, Hemigrapsus
sanguineus. Carcinus maenas did not differ significantly in the amount of time to handle
L. saxatilis of differing scarring history, which may be explained by their species-specific
handling behavior, claw morphology, and evolutionary history. While sub-lethal injuries
in the form of shell scars can provide snails with a reduced risk of future predation, it is
important to investigate crabs’ species-specific handling behavior.
Introduction
Shell forming mollusks offer accessible traits that often have been examined as
evidence reflecting ecological and evolutionary changes over space or time (Vermeij
1987). Shell thickness is one of the most important features determining a mollusk’s
susceptibility to predation (Vermeij 1987) and scarring may change shell thickness
(Greenfield et al. 2002). Evidence of sublethal predation in prey populations is often
67
associated with an increased risk of lethal predation (Geller 1990b, Meyer and Byers
2005), which is a generally accepted notion among researchers. However, this
phenomenon does not always occur in nature. Greenfield et al. (2002) found that shell
scars, from previous crab predation events, provide marsh snails, Littoraria irrorata, with
a reduced risk of predation from blue crabs, Callinectes sapidus. While a scar may
weaken a shell by compromising its structural integrity, scar repair may protect the snails
from predation by creating a thicker shell at the site of the wound (see Chapter I; Figure
1.13). That such prolonged handling time does decrease predator success has been shown
for the predator, C. sapidus, with its prey the snail, Littoraria irrorata in marshes on the
southern Atlantic coast of the U.S. (Greenfield et al. 2002). I test this observation for a
northern rocky intertidal system with the snail Littorina saxatilis and the crab predators,
Hemigrapsus sanguineus and Carcinus maenas. While the southern system (Littoraria
irrorata—Callinectes sapidus) consists of a pair of native prey and native predator, the
northern system consists of a native prey and two non-native predators, one of which
(Carcinus maenas), however, has had a long evolutionary history with the prey (Littorina
saxatilis) in Europe.
Blundon and Vermeij (1983) found that scarred and unscarred Littoraria irrorata, the
marsh periwinkle, were equally resistant to crushing forces, however the method crabs
typically use to eat Littorina saxatilis is peeling rather than crushing, unless the snail is
very small in comparison to the crab size (personal observation, Bertness and
Cunningham 1981). Thus, if snails had thicker shells at the scar, as scarred snails
typically do (see Chapter I, Figure 1.12 and 1.13), then as the crab peels away the shell to
the point of the scar, the crab may spend more time attempting to crack this portion and
68
thus potentially give up the attack. The longer it takes a predator to eat a prey item, the
less likely the predator will be successful (Rilov et al. 2004). Here, I investigate whether
sublethal predation may in fact provide prey with a reduced risk of lethal predation.
Objectives
I investigated the morphology of the native rocky shore snail, Littorina saxatilis, and
the snail’s differential vulnerability to predation based on its scarring history. To
understand how scarring in snails influences the success of predation by crabs,
observations of crabs handling both scarred and unscarred snails were made. I
hypothesized that crabs take longer to handle scarred snails and that as a result of this,
scarring provides these snails with a reduced risk of successful predation encounters.
Methods
To understand how scarring in snails influences the success of predation by crabs,
observations of crabs handling both scarred and unscarred snails were made. All
specimens were collected from southern New England, where Carcinus maenas and
Hemigrapsus sanguineus have overlapped for the longest period of time. Collections
were made from a rocky-intertidal site where Littorina saxatilis and the crab predators C.
maenas and H. sanguineus are all consistently abundant (Weekapaug Point, RI). Since H.
sanguineus are in such greater abundance than C. maenas in the intertidal zone at
Weekapaug Point, H. sanguineus were collected by hand, and C. maenas were collected
using minnow traps baited with canned cat food.
69
Morphometric analyses of snail height and crab carapace width were performed in
order to properly match crabs with snails, based on preliminary predation studies offering
snails of different sizes to crabs (Teck, unpublished data). On average, H. sanguineus
were 27.6 +/-0.60 mm SE in CW matched with snails 7.7 +/-0.41 mm SE in height, and
C. maenas were 49.3 +/-1.31 mm SE in CW matched with snails 8.4 +/-0.35 mm SE in
height. While the experimental animals may have differed in average CW, both species
were successful in eating snails of similar sizes (Appendix F, Figure F.2).
After 48 hours of acclimation to room temperature and laboratory conditions (30 ppt
seawater in tanks with aeration and pump filters) experimental trials began in a laboratory
dark room under a red light. Trials were performed in aquaria filled with 2.25 L of 30-35
ppt aerated seawater. Individual crabs (starved for at least 48 hours) were observed when
presented simultaneously with two snails matched for height, one unscarred snail and one
scarred snail. During each trial the crab’s handling of each snail was timed until both
snails were entirely consumed or for one hour (whichever came first). Trials were
discontinued if crabs did not touch the snails within the first 10 minutes. Only handling
times for 27 out of 94 snails were analyzed because crabs left many snails untouched,
unchipped, or uneaten (see Appendix F, Table F.1 for more details on the trial outcomes).
Handling times for each snail that was consumed first were compared across species and
scarring history. Handling times were log transformed for normality. All analyses were
performed with the statistical software JMP 5.1.
70
Results
Hemigrapsus sanguineus on average took longer to handle and consume scarred over
unscarred L. saxatilis (Figure 3.1, Table 3.1; ANOVA: F1,14=6.0, P=0.0297). However,
Carcinus maenas did not differ significantly in the amount of time to handle and
consume scarred and unscarred L. saxatilis (Figure 3.1, Table 3.1; ANOVA: F1,11=1.9,
P=0.1993). Although there is a trend for C. maenas to take longer to handle scarred over
unscarred snails, there is high variance in handling times, especially for scarred snails
(Table 3.1). Also, I expected a decreasing relationship between handling time and crab to
snail size ratio. I did not see this trend most likely because I did not include trials with
very small crab to snail size ratios (Appendix F: Table F.2, Figure F.1).
Discussion
Scarring provides historical evidence of failed predation events, revealing a level of
resistance by the prey to shell-breaking predators. Greenfield et al. (2002) suggest that
scarred marsh snails initially may be thicker than unscarred snails, and the scar is
verification that natural selection is in fact taking place in the system (Vermeij 1982b).
General snail shell thickening could occur as an inducible defense either from crabs
directly handling snails or from crab cues present in the water (Appleton and Palmer
1988, Etter 1988, Trussell 1996). However, the scar itself is thicker than the surrounding
shell, so the mechanism of healing from a crab’s failed attack results in a portion of the
shell that is more difficult to crack than the rest of the shell (see Chapter I). Overall
thickening of a snail shell provides snails with a reduced risk of predation (Chapter II),
71
and here, I show that localized thickening of the shell at a scar also may provide
protection for a snail.
Hemigrapsus sanguineus take longer to handle L. saxatilis with shell scars than those
without scars. The thicker portion of the shell at the scar lengthens the handling time for
H. sanguineus; as predicted, it is more difficult for these crabs to handle and consume
scarred snails over unscarred snails. Thus, these scarred snails are in fact less vulnerable
to predation by this species of crab, as increased handling time reduces the success of
predation (Rilov et al. 2004). Additionally, in Southern New England, where this study
was conducted, H. sanguineus is likely the principal predator that this snail will come
into contact with (personal observation, Lohrer and Whitlatch 2002a). The fortification of
the shell provided by the scar, however, does not always decrease the vulnerability of the
snail, since each crab species likely handles the snails in a different manner.
Carcinus maenas did not spend more time handling and consuming scarred snails
over unscarred snails, so the thickening of the shell at the scar did not prove to prolong
the act of predation consistently. The scar likely protects the snail best when a crab uses
the peeling technique of chipping away at the aperture over simply crushing the shell.
Crabs typically use the crushing technique when the crab to snail size ratio is high; as the
crab to snail size ratio decreases, a crab is more likely to have trouble crushing the shell
and will switch to peeling the snail shell at the aperture (Bertness and Cunningham
1981). However, there is a trend for C. maenas to take longer to handle the scarred snails
over the unscarred snails, but there is high variability in these handling times. Since I
have quite low power for this study (0.24), it is likely that with a higher sample size there
may be differences in C. maenas handling times for the scarred versus unscarred snails.
72
Both crab species in my experimental trials appeared to use the peeling technique
more often than the crushing technique. When the peeling technique is applied with one
claw, the other claw often holds the major whorl with a firm grip. As a result of this grip,
sometimes crabs of either species will puncture a hole into the whorl, even if they are not
successful at chipping away at the aperture. A few trials thus ended with a snail shell
intact apart from a hole in the body whorl caused by this behavior. Perhaps, C. maenas
despite its similar sized claws to H. sanguineus did use the crushing technique more often
than the peeling technique; larger crabs preying on relatively smaller snails will, as noted,
likely crush the entire shell, and the potential role of a thickened scar would thus be
eliminated or reduced substantially.
Hemigrapsus sanguineus has occurred on the southern New England coast only since
1993, whereas Carcinus maenas has been in the same region since around 1800. The
differences observed in handling scarred versus unscarred shells may be due to the much
longer experience C. maenas has had with L. saxatilis (both in North America and
Europe). In Georgia, U.S., scarred marsh snails Littoraria irrorata were thicker at the
aperture and were chosen less frequently over unscarred conspecifics by the predator
Callinectes sapidus (Greenfield et al. 2002). In shores further north, in Rhode Island,
Littorina saxatilis also had thicker scarred individuals, however, contrary to the
Littoraria irrorata-Callinectes sapidus pattern, the crab with the longest co-occurring
history with Littorina saxatilis, Carcinus maenas, did not take longer to consume scarred
versus unscarred snails. Perhaps the difference in thickness between scarred and
unscarred marsh snails is much greater than differences between scarred and unscarred L.
saxatilis in rocky shores further north. The healing process may produce a thicker scar
73
further south due to a greater ability to accrete calcium carbonate in the warmer waters
(Vermeij and Currey 1980); and thus Callinectes sapidus in the south may not have
developed the ability to overcome such great irregularities in shell thickness. Differences
in scarred and unscarred predation success may also be due to differences in claw
anatomy of the predators.
While these studies highlight the importance of recognizing the intricate variability
both within prey species and within predator species, conclusions can only be tentative as
trends would need to be examined with a higher sample size. The behavior of shellbreaking predators may appear similar (i.e. they may appear to use the same techniques),
but behavior cannot be generalized across a crab guild—individual species must be
examined to see if the application of a particular behavior yields similar results across
species. Finally, investigating interactions between non-native predators and native prey
elucidates the role of time in mediating interactions between these guilds.
74
Table 3.1 Scarred (S) versus unscarred (US) snails’ handling times for C. maenas (CM)
and H. sanguineus (HS), all crabs were only used once.
CM
HS
US
S
US
S
Mean
Handling
Time
(sec)
129.0
343.8
102.9
200.0
Log
(Mean
Handling
Time)
2.00
2.33
1.95
2.28
SE Log
(Mean
Handling
Time)
0.122
0.224
0.075
0.080
N
(number of
crabs)
7
5
11
4
5
F ratio
P
F1,11=1.9
0.1993
F1,14=6.0
0.0297
Scarred
500
Handling Time (sec)
ANOVA
Unscarred
400
300
200
4
7
11
100
0
Unscarred
Scarred
C. maenas
Unscarred
Scarred
H. sanguineus
Figure 3.1 Carcinus maenas did not differ significantly in the amount of time (sec +/SE) to handle and consume scarred and unscarred L. saxatilis (ANOVA: F1,11=1.9,
P=0.1993, power=0.24). Hemigrapsus sanguineus on average took longer to handle and
consume scarred over unscarred L. saxatilis (ANOVA: F1,14=6.0, P=0.0297). Both
species took the same amount of time to handle unscarred snails (ANOVA: F1,17=0.16,
P=0.6911) and scarred snails (ANOVA: F1,8=0.03, P=0.8583). The sample size (number
of crabs) for each average is shown above each bar, and crabs were only used once.
Handling times were log transformed for normality but presented in the figure with
untransformed handling times.
75
SUMMARY
In biogeographical studies phenotypic variation is often examined across a latitudinal
gradient. This is an example of a cline, or a gradual variation of a phenotype in a species
across a landscape. Clines can occur over large geographic scales, such as latitude, or
they can occur on smaller scales, such as across a tidal gradient. Through selection,
species shift their morphology and behavior as a result of a spectrum of abiotic and biotic
stresses that vary over time and space. Shell forming mollusks serve as a good model
because they often express ecological and evolutionary changes over time and space in
conspicuous ways such as shell morphology. Additionally, species that have limited
genetic mixing based on their reproductive cycle and low rate of dispersal often have
highly variable phenotypes among separate populations as the result of local adaptation.
One variation commonly seen across shelled mollusks is that shells tend to get thinner
in higher latitudes. Specifically for shell thickness, this negative relationship with latitude
is largely influenced by temperature and predation intensity. It is more difficult to accrete
calcium carbonate (shell) in colder waters, and shell-breaking predators can influence the
adaptation of shell form in a variety of ways. In the example of shell thickness, when
there are fewer predators there is a reduced pressure for the selection of thick shells.
Additionally, there tends to be fewer predators in these colder waters, so in this case,
temperature is also closely influencing predation intensity.
In general, many gastropods such as Nucella and littorinids express some similar
phenotypic patterns in several species and in various locations. When there is low wave
76
exposure, there tends to be a higher predation pressure because it is easier for crabs to
inhabit a wave protected site. Snails in these locations tend to have thicker shells, with
lower spire heights, and smaller apertures in response to this higher predation pressure,
while snails at sites with high wave exposure and thus lower predation pressure, likely
will have thinner shells, higher spires and larger apertures. Also, snails would need to
have a larger foot in these wave exposed sites in order to more firmly grasp to the
substrate. However, this regime may change at different temperatures and habitats. A site
may have high predation pressure and low wave exposure, but at cold temperatures it
may be difficult for snails to accrete calcium carbonate to thicken their shells.
The objective of my research was to explore a potential cline in a shell-forming
mollusk, Littorina saxatilis, across a dynamic landscape. The model I used is unique
because I examined a native mollusk species overlapping with two non-native predatory
crabs, whereas earlier studies have investigated native predation pressure influencing
shell shape. I found that clinal variation in shell morphology exists across the New
England range of Littorina saxatilis, and this variation is likely influenced largely by
historical differences in crab abundance and temperature. Snails in the north have shell
characteristics that make them more vulnerable to shell-breaking predators than those
from the south. The frequency of scarred snails at a site decreases as shell thickness
decreases largely due to the fact that shell-breaking predators are more often successful in
cracking thinner shells. Furthermore, those snails that are left scarred from failed
predation attempts may have protection from future predation events, due to localized
shell thickening at the healed over scar.
77
In the future, it would be interesting to test whether these differences in morphology
across New England are the result of genetic differences or phenotypic plasticity or a
combination of the two. Carcinus maenas has influenced shell shape in Littorina
obtusata in New England; before 1900, snails were thinner with higher spires than those
collected more recently among greater crab populations (Seeley 1986). Additionally, the
shells of northern New England snails are much thinner and weaker than southern New
England snails because predators tend to be more abundant further south, and Littorina
obtusata have been shown to exhibit phenotypic plasticity in shell shape in the presence
of chemical cues from predators (Trussell 1996). Freeman and Byers (2006) showed that
in the short time period H. sanguineus has been present in southern New England, the
mussel Mytilus edulis showed rapid evolution of inducible defenses when exposed to
chemical cues from this new predator, while mussels in areas further north did not show
inducible defenses when exposed to H. sanguineus (and H. sanguineus is not established
yet in these northern areas). However, both northern and southern populations responded
to the older invader, C. maenas which is present in both places. Thus, other prey, such as
Littorina saxatilis, are likely to exhibit similar species-specific responses to predators,
and these responses can rapidly evolve based on the intensity and composition of
predators present in the system.
Currently, C. maenas populations are patchy over time and space among sites in
Downeast Maine. However, as temperatures rise, C. maenas will likely become more
abundant in areas further north, posing a threat to local thin-shelled L. saxatilis
populations. Additionally, it is likely that with these warmer waters, H. sanguineus will
begin to extend its range into these northern areas, and it would be interesting to see how
78
both predators might influence shell shape. Although in my 2005 survey, the Southern
region (NH and southern ME coast) had only 19% H. sanguineus versus C. maenas, this
is a great increase since 2001, when H. sanguineus was reportedly not established yet in
this region (Tyrrell 1999, 2002).
Abiotic and biotic factors, which influence the cline in shell morphology, are not only
changing across the landscape and shifting with seasonal patterns, but also they are
changing over time as the result of two major issues currently affecting the study of
biogeography. With global climate change, temperature may be modified causing a shift
in many aspects of the community including the abundance of predators in the system.
Additionally, taking into account that two of these predators are non-native species also
influences the intensity of their presence in the system. Understanding the impact of
specific introduced species that may cause shifts in predator-prey dynamics and
community structure is crucial as communities become increasingly rich with non-native
species.
Future Directions
To help explain why clinal variation exists in L. saxatilis, I will outline six studies
that could be applied to L. saxatilis (or any species). (1) Lab experiments: one could
expose individuals from the same broods to varying regimes of temperature, predator
presence, and composition, and then measure shell shape and growth. (2) Observational
data: one could examine both phenotypic and genetic clines, examine the gene flow and
dispersal of L. saxatilis, look at detailed abiotic data (such as oceanographic patterns and
microclimate temperature (air and water temperature) in the intertidal zone), and examine
79
long term predator intensity and composition. (3) Transplant experiments: transplant
individuals from the same broods to a new location and compare their morphology to
individuals left in original location (this study could be performed in the lab to avoid
causing any genetic mixing in the field). (4) Tethering experiments: tether snails in the
intertidal zone at various tidal heights in order to compare predation rates and evidence of
unsuccessful predation attempts on snails of different origins in various locations. (5)
Biogeography: looking at both invasive and native ranges of a species, one could
examine how the species differs across varying environments of multiple ranges.
Consider how long the species has occurred in each area. For example, how do L.
saxatilis in New England differ from L. saxatilis in San Francisco Bay? Although L.
saxatilis in San Francisco Bay have been traced back to New England genetically
(Carlton and Cohen 1998), have their exposure to differing temperature and predator
regimes caused differences in shell shape between the two regions? One could also look
at congeners (such as considering the guild of Littorinids in an area) and compare guilds
across a variety of ranges. (6) Historical data: one could compare shells from museum
collections during different time periods for shell shape and shell damage and consider
how the predator and temperature regimes have changed over time. For example,
littorinids in New England were first exposed to the predator combination of native crabs
and lobsters prior to 1800, then C. maenas was added to the system in around 1817, and
then in 1988, H. sanguineus joined the predator guild. In the future, an additional nonnative species may become established in the region, for example, H. penicillatus could
arrive from Europe. How will this new predator regime within a likely altered future
temperature regime influence shell form in L. saxatilis?
80
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90
APPENDICES
91
APPENDIX A
Field Site Details
Table A.1 Site names and codes.
Region
Site Name
Stonington Point, CT
Weekapaug Point, RI
Rye Harbor, NH
Southern
Odiornes Point, NH
Kittery Point, ME
Evergreen site in Winter Harbor, ME
Frazer site in Winter Harbor, ME
Northern
Wonsqueak site in Winter Harbor, ME
Mermaid's Purse site in Prospect Harbor, ME
Comstock Point, ME
Eastport Harbor, ME
Northernmost Passamaquoddy site near Pleasant Point, ME
West Quoddy Head, ME
Wilbur Neck, ME (near Pembroke, ME)
Southernmost
Site
Code
SP
WP
RH
OP
KP
EV
FR
WO
MP
CP
EA
PA
QU
WN
92
Table A.2 Individual sites’ coordinates.
Region
Site
Code
Latitude Longitude
(°N)
(°W)
41.33
41.33
43.00
43.04
43.10
44.34
44.36
44.38
44.40
44.82
44.89
44.90
44.90
44.95
SP
WP
RH
Southern
OP
KP
EV
WO
Northern
FR
MP
CP
EA
Northernmost PA
QU
WN
Southernmost
-71.91
-71.75
-70.74
-70.71
-70.66
-68.05
-68.05
-68.07
-68.02
-66.95
-67.02
-66.99
-67.15
-67.04
Table A.3 Individual sites’ mean shell height and thickness (mm) (+/- SE) across all
sampled snails (N=number of observations).
Region
Site
Code
SP
WP
RH
Southern
OP
KP
EV
FR
Northern
WO
MP
CP
EA
Northernmost PA
QU
WN
Southernmost
N
182
200
121
167
140
115
123
228
263
107
159
101
100
172
Shell Height
Shell Thickness
(mm)
(mm)
mean +/-SE
mean
+/-SE
7.41
0.180
0.30
0.010
8.76
0.167
0.39
0.011
11.08
0.171
0.50
0.018
9.54
0.149
0.47
0.008
9.45
0.138
0.32
0.011
9.62
0.296
0.32
0.016
6.63
0.186
0.22
0.008
8.75
0.205
0.31
0.012
7.67
0.162
0.26
0.008
6.68
0.169
0.12
0.005
7.11
0.151
0.11
0.004
6.75
0.170
0.09
0.004
8.64
0.202
0.23
0.011
6.57
0.067
0.16
0.003
93
Table A.4 Individual sites differences in log transformed mean crab density (per 0.5 m2
quadrat), comparison for all pairs using Tukey-Kramer HSD. Levels not connected by
same letter are significantly different (α=0.05).
Region
Site
Southernmost
Southernmost
Southern
Northernmost
Northernmost
Northern
Southern
Northern
Northern
Southern
Northernmost
Northernmost
Northernmost
WP
SP
OP
PA
EA
FR
KP
WO
MP
RH
CP
WN
QU
Log
(Mean Crabs
per 0.5m2)
1.63
1.00
0.75
0.57
0.40
0.37
0.29
0.29
0.19
0.16
0.10
0.06
0.04
A
B
B
C
C
C
D
D
D
D
E
E
E
E
E
E
E
F
F
F
F
F
F
F
F
94
Eastport, ME
Bar Harbor, ME
Portsmouth Harbor, NH
New London, CT
Temperature (oC)
25
20
15
10
5
0
Jan
Apr
May
Jul
Aug
Oct
Dec
Figure A.1 Average monthly temperatures for four sites closest to the four regions
examined. I compiled data obtained from the National Oceanographic Data Center:
NOAA/National Ocean Service (NOS) tide stations and NOAA/National Data Buoy
Center (NDBC) buoys; “average water temperatures were computed from long-period
records ranging from several years to several decades depending on how long
observations have been taken at a given station”
(http://www.nodc.noaa.gov/dsdt/cwtg/aboutCWTG.html ). Thus, these temperatures are
conservative averages bearing in mind more recent increasing seawater temperatures.
During the months of April through October, bi-monthly averages are shown.
95
APPENDIX B
Aperture Thickness (mm)
Shell Thickness Comparisons between Field Sites within each Region
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
SP
WP
0
5
10
15
20
Height (mm)
Figure B.1 Comparison of sites within the Southernmost region: Weekapaug Point (WP)
has significantly thicker shells (0.39 +/- 0.011 mm SE) than Stonington Point (SP) (0.30
+/- 0.010 mm SE) (ANCOVA: F3,274=55.3, P=0.0091), and there is no interaction
between the covariate, height, and site (thickness measurements were log transformed for
statistical analyses but left untransformed for graphical purposes). Only snails between 5
and 10 mm in height were included in the above analyses.
96
Aperture Thickness (mm)
1.2
OP
1
RH
0.8
KP
0.6
0.4
0.2
0
0
5
10
15
20
Height (mm)
Figure B.2 Comparison of sites within the Southern region: Rye Harbor (RH) has
significantly thicker shells (0.50 +/- 0.018 mm SE) than Odiornes Point (OP), which has
significantly thicker shells (0.47 +/- 0.008 mm SE) than Kittery Point (KP) (0.32 +/0.011 mm SE) (ANCOVA: F5,238=65.1, P<0.0001; LS Means Differences Tukey HSD:
α=0.05). All three sites have significantly different thicknesses from one another, and
there is no interaction between the covariate, height, and site (thickness measurements
were log transformed for statistical analyses but left untransformed for graphical
purposes). Only snails between 5 and 10 mm in height were included in the above
analyses.
97
Aperture Thickness (mm)
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
EV
FR
WO
MP
0
5
10
15
20
Height (mm)
Figure B.3 There are no significant differences among the sites within the Northern
region: Evergreen (EV) (0.32 +/- 0.016 mm SE), Frazer (FR) (0.22 +/- 0.008 mm SE),
Wonsqueak (WO) (0.31 +/- 0.012 mm SE), and Mermaid's Purse (MP) (0.26 +/- 0.008
mm SE) (ANCOVA: F7,397=26.2, P=0.0664). There is no interaction between the
covariate, height, and site (thickness measurements were log transformed for statistical
analyses but left untransformed for graphical purposes). Only snails between 5 and 10
mm in height were included in the above analyses.
98
Aperture Thickness (mm)
0.8
CP
0.7
EA
0.6
PA
0.5
QU
0.4
WN
0.3
0.2
0.1
0
0
5
10
15
20
Height (mm)
Figure B.4 All sites within the Northernmost site are significantly different from one
another in shell thickness except for the two thickest-shelled sites: West Quoddy Head
(QU) (0.23 +/- 0.011 mm SE) and Wilbur Neck (WN) (0.16 +/- 0.003 mm SE). The other
three sites are significantly different from one another and from the two thickest-shelled
sites: Passamaquoddy (PA) (0.09 +/- 0.004 mm SE), Eastport Harbor (EA) (0.11 +/0.004 mm SE), and Comstock Point (CP) (0.12 +/- 0.005 mm SE) (ANCOVA:
F9,535=47.4; LS Means Differences Tukey HSD, α=0.05). There is an interaction between
the covariate, height, and site (thickness measurements were log transformed for
statistical analyses but left untransformed for graphical purposes). Only snails between 5
and 10 mm in height were included in the above analyses.
99
APPENDIX C
Additional Results Comparing Crab Predation of Snails Differing in Thickness
Results for 36 additional trials are as follows (these data were not used in analyses):
(A) Eight crabs ate neither snail within an hour (during 14 trials, 1-3 trials for each crab)
(B) Three crabs ate both snails between observations (during three trials)
(C) Four crabs ate both snails between observations (during five trials) after eating one
in a previous trial (previous trials included in data analysis)
(D) Three crabs ate neither snail within an hour (during four trials) and in a subsequent
trial were observed to choose one snail first (subsequent trials included in data analysis)
(E) During 10 trials, crabs were observed eating one snail, but these trials were
eliminated because these crabs were already observed eating one snail in a previous trial
(previous trials included in data analysis). During 9 of these trials, the WN snail was
eaten first over the OP snail.
100
APPENDIX D
Mean distance (mm)
Comparison of Odiornes Point versus Wilbur Neck Snail Movement
OP
200
WN
150
100
50
0
0
1
2
3
4
5
6
7
8
9
10
11
Time (minutes)
Figure D.1 Comparison of snail movement from Northeastern Maine and New
Hampshire. Snails from Wilbur Neck, Northeastern Maine (WN) (open diamonds) moved
significantly further (276 mm) than those from Odiornes Point, New Hampshire (OP)
(filled diamonds) (112.5 mm) (repeated measures ANOVA: P=0.004). Movement was
observed (mm +/-SE) after snails (6 mm ±0.5) were placed in the center of a Plexiglas
arena in 5 cm of seawater. For each site the mean distance 10 snails moved during five 10
minute trials is shown. All trials were performed in August 2004 at the UNH Coastal Lab
in Newcastle, NH after all snails had been acclimated to ambient NH seawater
temperatures for 7 weeks.
101
APPENDIX E
Comparing Claw Sizes between Hemigrapsus sanguineus and Carcinus maenas
B
A
Figure E.1 Diagram of claw measured to calculate claw area: (A) length and (B) height.
Claw Area (mm)
2
600
H. sanguineus
500
C. maenas
400
300
200
100
0
20
30
40
50
Carapace Width (mm)
Figure E.2 Comparing claw sizes between two crab species: Hemigrapsus sanguineus
(filled circles, solid line) and Carcinus maenas (open circles, dashed line) with similar
claw areas (t=0.518, DF=29, P=0.6087), differ significantly in carapace width (t=14.490,
DF=20.3, P<0.0001); small H. sanguineus (28.6 +/- 0.79 mm CW) have similar claw
areas (264.6 +/- 20.58 and 280.3 +/- 16.1, respectively) to larger C. maenas (48.1 +/- 1.09
mm CW). Crabs measured here were used in predation trials with Littorina saxatilis
(Chapter III). Claw areas were calculated by multiplying average claw length by average
claw height per crab as shown in Figure E.1; claw areas were normally distributed.
102
APPENDIX F
Handling Time Details for Trials with Carcinus maenas and Hemigrapsus
sanguineus Preying upon Scarred and Unscarred Snails
Table F.1a Outcomes of 124 handling time trials for C. maenas (CM) and H. sanguineus
(HS) preying upon scarred (S) and unscarred (US) snails matched for height (some of
these trials used crabs more than once).
Outcome
Handled
none
both
both
both
both
both
both
both
both
both
S only
S only
S only
US only
US only
US only
Chipped
none
US only
S only
both
none
S only
US only
none
none
none
none
S only
none
none
US only
none
Eaten
none
none
none
none
none
US only
S only
US only
S only
both
none
none
S only
none
none
US only
Total number of trials: 124
Number of
trials
HS
CM
29
3
1
3
3
1
1
2
4
10
6
2
0
0
0
0
1
2
21
19
1
1
3
1
3
0
1
1
1
0
3
1
78
46
103
Table F1.b Choice results for trials with C. maenas (CM) and H. sanguineus (HS)
during which both unscarred (US) or scarred (S) snails were handled (some of these trials
used crabs more than once).
CM
HS
Ate only US
Ate US 1st over S
2
10
6
13
Total US eaten
12
19
Ate only S
Ate S 1st over US
2
9
1
8
Total S eaten
11
9
Table F.2 There were no significant relationships between log transformed handling
time and crab to snail size ratio for unscarred and scarred snails handled by both C.
maenas and H. sanguineus (some of these trials used crabs more than once, so analyses
were blocked by crab identity) (see Figure F.1).
Linear Fit
CM
HS
US
S
US
S
y=-0.0188x + 2.24
y=-0.1895x + 3.43
y=-0.0227x + 2.16
y=-0.0245x + 2.46
R2
0.00079
0.09
0.00045
0.0017
ANOVA
F ratio
P
F1,12=0.0087
F1,10=0.89
F1,21=0.0090
F1,11=0.0168
0.9274
0.3711
0.9253
0.8993
104
Handling Time (sec)
1000
CM US
CM S
800
HS US
HS S
600
400
200
0
2.5
3.5
4.5
5.5
6.5
7.5
Crab to Snail Size Ratio
Figure F.1 Snail handling time is plotted against crab to snail size ratio. Carcinus
maenas (CM) is plotted with squares; Hemigrapsus sanguineus (HS) is plotted with
diamonds; unscarred snails (US) are open shapes, and scarred (S) snails are filled shapes
(some of these trials used crabs more than once) (see Table A.2). Analyses were blocked
by crab identity and handling times were long transformed (but left untransformed for
graphical purposes).
105
HS
Snail Height (mm)
10.5
CM
9.5
8.5
7.5
6.5
5.5
22
32
42
52
Crab Carapace Width (mm)
Figure F.2 Snails were presented to crabs and successfully consumed. Snail heights of
consumed snails are plotted against the carapace widths of the crabs that ate the snails.
Carcinus maenas (CM) is plotted with open diamonds; Hemigrapsus sanguineus (HS) is
plotted with filled diamonds. Since I chose which snails would be presented to which
crabs unsystematically and individual crabs appear more than once in the figure, it is not
appropriate to compare the average snail heights successfully eaten by each species. The
figure simply shows that the two species of crabs overlap in the size of snails they can
eat, despite their difference in carapace width.
106